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![Lai Jiangshan](/assets/img/avatar_default.png)
This commit implements an SRCU state machine in support of call_srcu(). The state machine is preemptible, light-weight, and single-threaded, minimizing synchronization overhead. In particular, there is no longer any need for synchronize_srcu() to be guarded by a mutex. Expedited processing is handled, at least in the absence of concurrent grace-period operations on that same srcu_struct structure, by having the synchronize_srcu_expedited() thread take on the role of the workqueue thread for one iteration. There is a reasonable probability that a given SRCU callback will be invoked on the same CPU that registered it, however, there is no guarantee. Concurrent SRCU grace-period primitives can cause callbacks to be executed elsewhere, even in absence of CPU-hotplug operations. Callbacks execute in process context, but under the influence of local_bh_disable(), so it is illegal to sleep in an SRCU callback function. Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
650 lines
20 KiB
C
650 lines
20 KiB
C
/*
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* Sleepable Read-Copy Update mechanism for mutual exclusion.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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*
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* Copyright (C) IBM Corporation, 2006
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*
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* Author: Paul McKenney <paulmck@us.ibm.com>
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*
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* For detailed explanation of Read-Copy Update mechanism see -
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* Documentation/RCU/ *.txt
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*
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*/
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#include <linux/export.h>
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#include <linux/mutex.h>
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#include <linux/percpu.h>
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#include <linux/preempt.h>
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#include <linux/rcupdate.h>
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#include <linux/sched.h>
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#include <linux/smp.h>
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#include <linux/delay.h>
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#include <linux/srcu.h>
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/*
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* Initialize an rcu_batch structure to empty.
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*/
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static inline void rcu_batch_init(struct rcu_batch *b)
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{
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b->head = NULL;
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b->tail = &b->head;
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}
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/*
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* Enqueue a callback onto the tail of the specified rcu_batch structure.
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*/
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static inline void rcu_batch_queue(struct rcu_batch *b, struct rcu_head *head)
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{
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*b->tail = head;
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b->tail = &head->next;
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}
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/*
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* Is the specified rcu_batch structure empty?
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*/
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static inline bool rcu_batch_empty(struct rcu_batch *b)
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{
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return b->tail == &b->head;
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}
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/*
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* Remove the callback at the head of the specified rcu_batch structure
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* and return a pointer to it, or return NULL if the structure is empty.
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*/
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static inline struct rcu_head *rcu_batch_dequeue(struct rcu_batch *b)
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{
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struct rcu_head *head;
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if (rcu_batch_empty(b))
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return NULL;
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head = b->head;
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b->head = head->next;
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if (b->tail == &head->next)
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rcu_batch_init(b);
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return head;
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}
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/*
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* Move all callbacks from the rcu_batch structure specified by "from" to
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* the structure specified by "to".
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*/
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static inline void rcu_batch_move(struct rcu_batch *to, struct rcu_batch *from)
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{
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if (!rcu_batch_empty(from)) {
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*to->tail = from->head;
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to->tail = from->tail;
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rcu_batch_init(from);
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}
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}
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/* single-thread state-machine */
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static void process_srcu(struct work_struct *work);
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static int init_srcu_struct_fields(struct srcu_struct *sp)
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{
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sp->completed = 0;
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spin_lock_init(&sp->queue_lock);
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sp->running = false;
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rcu_batch_init(&sp->batch_queue);
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rcu_batch_init(&sp->batch_check0);
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rcu_batch_init(&sp->batch_check1);
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rcu_batch_init(&sp->batch_done);
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INIT_DELAYED_WORK(&sp->work, process_srcu);
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sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
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return sp->per_cpu_ref ? 0 : -ENOMEM;
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}
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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int __init_srcu_struct(struct srcu_struct *sp, const char *name,
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struct lock_class_key *key)
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{
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/* Don't re-initialize a lock while it is held. */
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debug_check_no_locks_freed((void *)sp, sizeof(*sp));
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lockdep_init_map(&sp->dep_map, name, key, 0);
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return init_srcu_struct_fields(sp);
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}
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EXPORT_SYMBOL_GPL(__init_srcu_struct);
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#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
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/**
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* init_srcu_struct - initialize a sleep-RCU structure
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* @sp: structure to initialize.
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*
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* Must invoke this on a given srcu_struct before passing that srcu_struct
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* to any other function. Each srcu_struct represents a separate domain
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* of SRCU protection.
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*/
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int init_srcu_struct(struct srcu_struct *sp)
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{
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return init_srcu_struct_fields(sp);
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}
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EXPORT_SYMBOL_GPL(init_srcu_struct);
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#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
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/*
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* Returns approximate total of the readers' ->seq[] values for the
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* rank of per-CPU counters specified by idx.
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*/
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static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
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{
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int cpu;
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unsigned long sum = 0;
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unsigned long t;
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for_each_possible_cpu(cpu) {
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t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
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sum += t;
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}
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return sum;
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}
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/*
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* Returns approximate number of readers active on the specified rank
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* of the per-CPU ->c[] counters.
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*/
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static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
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{
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int cpu;
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unsigned long sum = 0;
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unsigned long t;
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for_each_possible_cpu(cpu) {
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t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
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sum += t;
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}
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return sum;
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}
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/*
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* Return true if the number of pre-existing readers is determined to
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* be stably zero. An example unstable zero can occur if the call
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* to srcu_readers_active_idx() misses an __srcu_read_lock() increment,
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* but due to task migration, sees the corresponding __srcu_read_unlock()
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* decrement. This can happen because srcu_readers_active_idx() takes
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* time to sum the array, and might in fact be interrupted or preempted
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* partway through the summation.
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*/
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static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
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{
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unsigned long seq;
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seq = srcu_readers_seq_idx(sp, idx);
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/*
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* The following smp_mb() A pairs with the smp_mb() B located in
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* __srcu_read_lock(). This pairing ensures that if an
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* __srcu_read_lock() increments its counter after the summation
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* in srcu_readers_active_idx(), then the corresponding SRCU read-side
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* critical section will see any changes made prior to the start
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* of the current SRCU grace period.
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*
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* Also, if the above call to srcu_readers_seq_idx() saw the
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* increment of ->seq[], then the call to srcu_readers_active_idx()
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* must see the increment of ->c[].
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*/
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smp_mb(); /* A */
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/*
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* Note that srcu_readers_active_idx() can incorrectly return
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* zero even though there is a pre-existing reader throughout.
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* To see this, suppose that task A is in a very long SRCU
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* read-side critical section that started on CPU 0, and that
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* no other reader exists, so that the sum of the counters
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* is equal to one. Then suppose that task B starts executing
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* srcu_readers_active_idx(), summing up to CPU 1, and then that
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* task C starts reading on CPU 0, so that its increment is not
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* summed, but finishes reading on CPU 2, so that its decrement
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* -is- summed. Then when task B completes its sum, it will
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* incorrectly get zero, despite the fact that task A has been
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* in its SRCU read-side critical section the whole time.
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*
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* We therefore do a validation step should srcu_readers_active_idx()
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* return zero.
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*/
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if (srcu_readers_active_idx(sp, idx) != 0)
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return false;
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/*
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* The remainder of this function is the validation step.
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* The following smp_mb() D pairs with the smp_mb() C in
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* __srcu_read_unlock(). If the __srcu_read_unlock() was seen
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* by srcu_readers_active_idx() above, then any destructive
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* operation performed after the grace period will happen after
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* the corresponding SRCU read-side critical section.
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*
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* Note that there can be at most NR_CPUS worth of readers using
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* the old index, which is not enough to overflow even a 32-bit
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* integer. (Yes, this does mean that systems having more than
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* a billion or so CPUs need to be 64-bit systems.) Therefore,
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* the sum of the ->seq[] counters cannot possibly overflow.
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* Therefore, the only way that the return values of the two
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* calls to srcu_readers_seq_idx() can be equal is if there were
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* no increments of the corresponding rank of ->seq[] counts
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* in the interim. But the missed-increment scenario laid out
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* above includes an increment of the ->seq[] counter by
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* the corresponding __srcu_read_lock(). Therefore, if this
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* scenario occurs, the return values from the two calls to
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* srcu_readers_seq_idx() will differ, and thus the validation
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* step below suffices.
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*/
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smp_mb(); /* D */
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return srcu_readers_seq_idx(sp, idx) == seq;
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}
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/**
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* srcu_readers_active - returns approximate number of readers.
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* @sp: which srcu_struct to count active readers (holding srcu_read_lock).
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*
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* Note that this is not an atomic primitive, and can therefore suffer
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* severe errors when invoked on an active srcu_struct. That said, it
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* can be useful as an error check at cleanup time.
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*/
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static int srcu_readers_active(struct srcu_struct *sp)
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{
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int cpu;
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unsigned long sum = 0;
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for_each_possible_cpu(cpu) {
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sum += ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[0]);
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sum += ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[1]);
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}
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return sum;
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}
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/**
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* cleanup_srcu_struct - deconstruct a sleep-RCU structure
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* @sp: structure to clean up.
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*
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* Must invoke this after you are finished using a given srcu_struct that
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* was initialized via init_srcu_struct(), else you leak memory.
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*/
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void cleanup_srcu_struct(struct srcu_struct *sp)
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{
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int sum;
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sum = srcu_readers_active(sp);
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WARN_ON(sum); /* Leakage unless caller handles error. */
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if (sum != 0)
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return;
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free_percpu(sp->per_cpu_ref);
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sp->per_cpu_ref = NULL;
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}
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EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
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/*
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* Counts the new reader in the appropriate per-CPU element of the
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* srcu_struct. Must be called from process context.
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* Returns an index that must be passed to the matching srcu_read_unlock().
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*/
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int __srcu_read_lock(struct srcu_struct *sp)
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{
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int idx;
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preempt_disable();
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idx = rcu_dereference_index_check(sp->completed,
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rcu_read_lock_sched_held()) & 0x1;
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ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;
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smp_mb(); /* B */ /* Avoid leaking the critical section. */
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ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;
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preempt_enable();
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return idx;
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}
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EXPORT_SYMBOL_GPL(__srcu_read_lock);
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/*
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* Removes the count for the old reader from the appropriate per-CPU
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* element of the srcu_struct. Note that this may well be a different
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* CPU than that which was incremented by the corresponding srcu_read_lock().
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* Must be called from process context.
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*/
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void __srcu_read_unlock(struct srcu_struct *sp, int idx)
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{
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preempt_disable();
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smp_mb(); /* C */ /* Avoid leaking the critical section. */
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ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) -= 1;
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preempt_enable();
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}
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EXPORT_SYMBOL_GPL(__srcu_read_unlock);
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/*
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* We use an adaptive strategy for synchronize_srcu() and especially for
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* synchronize_srcu_expedited(). We spin for a fixed time period
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* (defined below) to allow SRCU readers to exit their read-side critical
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* sections. If there are still some readers after 10 microseconds,
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* we repeatedly block for 1-millisecond time periods. This approach
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* has done well in testing, so there is no need for a config parameter.
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*/
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#define SRCU_RETRY_CHECK_DELAY 5
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#define SYNCHRONIZE_SRCU_TRYCOUNT 2
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#define SYNCHRONIZE_SRCU_EXP_TRYCOUNT 12
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/*
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* @@@ Wait until all pre-existing readers complete. Such readers
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* will have used the index specified by "idx".
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* the caller should ensures the ->completed is not changed while checking
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* and idx = (->completed & 1) ^ 1
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*/
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static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
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{
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for (;;) {
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if (srcu_readers_active_idx_check(sp, idx))
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return true;
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if (--trycount <= 0)
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return false;
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udelay(SRCU_RETRY_CHECK_DELAY);
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}
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}
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/*
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* Increment the ->completed counter so that future SRCU readers will
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* use the other rank of the ->c[] and ->seq[] arrays. This allows
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* us to wait for pre-existing readers in a starvation-free manner.
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*/
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static void srcu_flip(struct srcu_struct *sp)
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{
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sp->completed++;
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}
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/*
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* Enqueue an SRCU callback on the specified srcu_struct structure,
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* initiating grace-period processing if it is not already running.
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*/
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void call_srcu(struct srcu_struct *sp, struct rcu_head *head,
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void (*func)(struct rcu_head *head))
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{
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unsigned long flags;
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head->next = NULL;
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head->func = func;
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spin_lock_irqsave(&sp->queue_lock, flags);
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rcu_batch_queue(&sp->batch_queue, head);
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if (!sp->running) {
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sp->running = true;
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queue_delayed_work(system_nrt_wq, &sp->work, 0);
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}
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spin_unlock_irqrestore(&sp->queue_lock, flags);
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}
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EXPORT_SYMBOL_GPL(call_srcu);
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struct rcu_synchronize {
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struct rcu_head head;
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struct completion completion;
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};
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/*
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* Awaken the corresponding synchronize_srcu() instance now that a
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* grace period has elapsed.
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*/
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static void wakeme_after_rcu(struct rcu_head *head)
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{
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struct rcu_synchronize *rcu;
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rcu = container_of(head, struct rcu_synchronize, head);
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complete(&rcu->completion);
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}
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static void srcu_advance_batches(struct srcu_struct *sp, int trycount);
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static void srcu_reschedule(struct srcu_struct *sp);
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/*
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* Helper function for synchronize_srcu() and synchronize_srcu_expedited().
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*/
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static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
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{
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struct rcu_synchronize rcu;
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struct rcu_head *head = &rcu.head;
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bool done = false;
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rcu_lockdep_assert(!lock_is_held(&sp->dep_map) &&
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!lock_is_held(&rcu_bh_lock_map) &&
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!lock_is_held(&rcu_lock_map) &&
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!lock_is_held(&rcu_sched_lock_map),
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"Illegal synchronize_srcu() in same-type SRCU (or RCU) read-side critical section");
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init_completion(&rcu.completion);
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head->next = NULL;
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head->func = wakeme_after_rcu;
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spin_lock_irq(&sp->queue_lock);
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if (!sp->running) {
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/* steal the processing owner */
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sp->running = true;
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rcu_batch_queue(&sp->batch_check0, head);
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spin_unlock_irq(&sp->queue_lock);
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srcu_advance_batches(sp, trycount);
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if (!rcu_batch_empty(&sp->batch_done)) {
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BUG_ON(sp->batch_done.head != head);
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rcu_batch_dequeue(&sp->batch_done);
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done = true;
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}
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/* give the processing owner to work_struct */
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srcu_reschedule(sp);
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} else {
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rcu_batch_queue(&sp->batch_queue, head);
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spin_unlock_irq(&sp->queue_lock);
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}
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if (!done)
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wait_for_completion(&rcu.completion);
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}
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/**
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* synchronize_srcu - wait for prior SRCU read-side critical-section completion
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* @sp: srcu_struct with which to synchronize.
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*
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* Flip the completed counter, and wait for the old count to drain to zero.
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* As with classic RCU, the updater must use some separate means of
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* synchronizing concurrent updates. Can block; must be called from
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* process context.
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*
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* Note that it is illegal to call synchronize_srcu() from the corresponding
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* SRCU read-side critical section; doing so will result in deadlock.
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* However, it is perfectly legal to call synchronize_srcu() on one
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* srcu_struct from some other srcu_struct's read-side critical section.
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*/
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void synchronize_srcu(struct srcu_struct *sp)
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{
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__synchronize_srcu(sp, SYNCHRONIZE_SRCU_TRYCOUNT);
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}
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EXPORT_SYMBOL_GPL(synchronize_srcu);
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/**
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* synchronize_srcu_expedited - Brute-force SRCU grace period
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* @sp: srcu_struct with which to synchronize.
|
|
*
|
|
* Wait for an SRCU grace period to elapse, but be more aggressive about
|
|
* spinning rather than blocking when waiting.
|
|
*
|
|
* Note that it is illegal to call this function while holding any lock
|
|
* that is acquired by a CPU-hotplug notifier. It is also illegal to call
|
|
* synchronize_srcu_expedited() from the corresponding SRCU read-side
|
|
* critical section; doing so will result in deadlock. However, it is
|
|
* perfectly legal to call synchronize_srcu_expedited() on one srcu_struct
|
|
* from some other srcu_struct's read-side critical section, as long as
|
|
* the resulting graph of srcu_structs is acyclic.
|
|
*/
|
|
void synchronize_srcu_expedited(struct srcu_struct *sp)
|
|
{
|
|
__synchronize_srcu(sp, SYNCHRONIZE_SRCU_EXP_TRYCOUNT);
|
|
}
|
|
EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
|
|
|
|
/**
|
|
* srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
|
|
*/
|
|
void srcu_barrier(struct srcu_struct *sp)
|
|
{
|
|
synchronize_srcu(sp);
|
|
}
|
|
EXPORT_SYMBOL_GPL(srcu_barrier);
|
|
|
|
/**
|
|
* srcu_batches_completed - return batches completed.
|
|
* @sp: srcu_struct on which to report batch completion.
|
|
*
|
|
* Report the number of batches, correlated with, but not necessarily
|
|
* precisely the same as, the number of grace periods that have elapsed.
|
|
*/
|
|
long srcu_batches_completed(struct srcu_struct *sp)
|
|
{
|
|
return sp->completed;
|
|
}
|
|
EXPORT_SYMBOL_GPL(srcu_batches_completed);
|
|
|
|
#define SRCU_CALLBACK_BATCH 10
|
|
#define SRCU_INTERVAL 1
|
|
|
|
/*
|
|
* Move any new SRCU callbacks to the first stage of the SRCU grace
|
|
* period pipeline.
|
|
*/
|
|
static void srcu_collect_new(struct srcu_struct *sp)
|
|
{
|
|
if (!rcu_batch_empty(&sp->batch_queue)) {
|
|
spin_lock_irq(&sp->queue_lock);
|
|
rcu_batch_move(&sp->batch_check0, &sp->batch_queue);
|
|
spin_unlock_irq(&sp->queue_lock);
|
|
}
|
|
}
|
|
|
|
/*
|
|
* Core SRCU state machine. Advance callbacks from ->batch_check0 to
|
|
* ->batch_check1 and then to ->batch_done as readers drain.
|
|
*/
|
|
static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
|
|
{
|
|
int idx = 1 ^ (sp->completed & 1);
|
|
|
|
/*
|
|
* Because readers might be delayed for an extended period after
|
|
* fetching ->completed for their index, at any point in time there
|
|
* might well be readers using both idx=0 and idx=1. We therefore
|
|
* need to wait for readers to clear from both index values before
|
|
* invoking a callback.
|
|
*/
|
|
|
|
if (rcu_batch_empty(&sp->batch_check0) &&
|
|
rcu_batch_empty(&sp->batch_check1))
|
|
return; /* no callbacks need to be advanced */
|
|
|
|
if (!try_check_zero(sp, idx, trycount))
|
|
return; /* failed to advance, will try after SRCU_INTERVAL */
|
|
|
|
/*
|
|
* The callbacks in ->batch_check1 have already done with their
|
|
* first zero check and flip back when they were enqueued on
|
|
* ->batch_check0 in a previous invocation of srcu_advance_batches().
|
|
* (Presumably try_check_zero() returned false during that
|
|
* invocation, leaving the callbacks stranded on ->batch_check1.)
|
|
* They are therefore ready to invoke, so move them to ->batch_done.
|
|
*/
|
|
rcu_batch_move(&sp->batch_done, &sp->batch_check1);
|
|
|
|
if (rcu_batch_empty(&sp->batch_check0))
|
|
return; /* no callbacks need to be advanced */
|
|
srcu_flip(sp);
|
|
|
|
/*
|
|
* The callbacks in ->batch_check0 just finished their
|
|
* first check zero and flip, so move them to ->batch_check1
|
|
* for future checking on the other idx.
|
|
*/
|
|
rcu_batch_move(&sp->batch_check1, &sp->batch_check0);
|
|
|
|
/*
|
|
* SRCU read-side critical sections are normally short, so check
|
|
* at least twice in quick succession after a flip.
|
|
*/
|
|
trycount = trycount < 2 ? 2 : trycount;
|
|
if (!try_check_zero(sp, idx^1, trycount))
|
|
return; /* failed to advance, will try after SRCU_INTERVAL */
|
|
|
|
/*
|
|
* The callbacks in ->batch_check1 have now waited for all
|
|
* pre-existing readers using both idx values. They are therefore
|
|
* ready to invoke, so move them to ->batch_done.
|
|
*/
|
|
rcu_batch_move(&sp->batch_done, &sp->batch_check1);
|
|
}
|
|
|
|
/*
|
|
* Invoke a limited number of SRCU callbacks that have passed through
|
|
* their grace period. If there are more to do, SRCU will reschedule
|
|
* the workqueue.
|
|
*/
|
|
static void srcu_invoke_callbacks(struct srcu_struct *sp)
|
|
{
|
|
int i;
|
|
struct rcu_head *head;
|
|
|
|
for (i = 0; i < SRCU_CALLBACK_BATCH; i++) {
|
|
head = rcu_batch_dequeue(&sp->batch_done);
|
|
if (!head)
|
|
break;
|
|
local_bh_disable();
|
|
head->func(head);
|
|
local_bh_enable();
|
|
}
|
|
}
|
|
|
|
/*
|
|
* Finished one round of SRCU grace period. Start another if there are
|
|
* more SRCU callbacks queued, otherwise put SRCU into not-running state.
|
|
*/
|
|
static void srcu_reschedule(struct srcu_struct *sp)
|
|
{
|
|
bool pending = true;
|
|
|
|
if (rcu_batch_empty(&sp->batch_done) &&
|
|
rcu_batch_empty(&sp->batch_check1) &&
|
|
rcu_batch_empty(&sp->batch_check0) &&
|
|
rcu_batch_empty(&sp->batch_queue)) {
|
|
spin_lock_irq(&sp->queue_lock);
|
|
if (rcu_batch_empty(&sp->batch_done) &&
|
|
rcu_batch_empty(&sp->batch_check1) &&
|
|
rcu_batch_empty(&sp->batch_check0) &&
|
|
rcu_batch_empty(&sp->batch_queue)) {
|
|
sp->running = false;
|
|
pending = false;
|
|
}
|
|
spin_unlock_irq(&sp->queue_lock);
|
|
}
|
|
|
|
if (pending)
|
|
queue_delayed_work(system_nrt_wq, &sp->work, SRCU_INTERVAL);
|
|
}
|
|
|
|
/*
|
|
* This is the work-queue function that handles SRCU grace periods.
|
|
*/
|
|
static void process_srcu(struct work_struct *work)
|
|
{
|
|
struct srcu_struct *sp;
|
|
|
|
sp = container_of(work, struct srcu_struct, work.work);
|
|
|
|
srcu_collect_new(sp);
|
|
srcu_advance_batches(sp, 1);
|
|
srcu_invoke_callbacks(sp);
|
|
srcu_reschedule(sp);
|
|
}
|