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![Paul E. McKenney](/assets/img/avatar_default.png)
Tiny SRCU readers can appear at task level, but also in interrupt and softirq handlers. Because Tiny SRCU is selected only in kernels built with CONFIG_SMP=n and CONFIG_PREEMPTION=n, it is not possible for a grace period to start while there is a non-task-level SRCU reader executing. This means that it does not make sense for __srcu_read_unlock() to awaken the Tiny SRCU grace period, because that can only happen when the grace period is waiting for one value of ->srcu_idx and __srcu_read_unlock() is ending the last reader for some other value of ->srcu_idx. After all, any such wakeup will be redundant. Worse yet, in some cases, such wakeups generate lockdep splats: ====================================================== WARNING: possible circular locking dependency detected 5.15.0-rc1+ #3758 Not tainted ------------------------------------------------------ rcu_torture_rea/53 is trying to acquire lock: ffffffff9514e6a8 (srcu_ctl.srcu_wq.lock){..-.}-{2:2}, at: xa/0x30 but task is already holding lock: ffff95c642479d80 (&p->pi_lock){-.-.}-{2:2}, at: _extend+0x370/0x400 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #1 (&p->pi_lock){-.-.}-{2:2}: _raw_spin_lock_irqsave+0x2f/0x50 try_to_wake_up+0x50/0x580 swake_up_locked.part.7+0xe/0x30 swake_up_one+0x22/0x30 rcutorture_one_extend+0x1b6/0x400 rcu_torture_one_read+0x290/0x5d0 rcu_torture_timer+0x1a/0x70 call_timer_fn+0xa6/0x230 run_timer_softirq+0x493/0x4c0 __do_softirq+0xc0/0x371 irq_exit+0x73/0x90 sysvec_apic_timer_interrupt+0x63/0x80 asm_sysvec_apic_timer_interrupt+0x12/0x20 default_idle+0xb/0x10 default_idle_call+0x5e/0x170 do_idle+0x18a/0x1f0 cpu_startup_entry+0xa/0x10 start_kernel+0x678/0x69f secondary_startup_64_no_verify+0xc2/0xcb -> #0 (srcu_ctl.srcu_wq.lock){..-.}-{2:2}: __lock_acquire+0x130c/0x2440 lock_acquire+0xc2/0x270 _raw_spin_lock_irqsave+0x2f/0x50 swake_up_one+0xa/0x30 rcutorture_one_extend+0x387/0x400 rcu_torture_one_read+0x290/0x5d0 rcu_torture_reader+0xac/0x200 kthread+0x12d/0x150 ret_from_fork+0x22/0x30 other info that might help us debug this: Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&p->pi_lock); lock(srcu_ctl.srcu_wq.lock); lock(&p->pi_lock); lock(srcu_ctl.srcu_wq.lock); *** DEADLOCK *** 1 lock held by rcu_torture_rea/53: #0: ffff95c642479d80 (&p->pi_lock){-.-.}-{2:2}, at: _extend+0x370/0x400 stack backtrace: CPU: 0 PID: 53 Comm: rcu_torture_rea Not tainted 5.15.0-rc1+ Hardware name: Red Hat KVM/RHEL-AV, BIOS e_el8.5.0+746+bbd5d70c 04/01/2014 Call Trace: check_noncircular+0xfe/0x110 ? find_held_lock+0x2d/0x90 __lock_acquire+0x130c/0x2440 lock_acquire+0xc2/0x270 ? swake_up_one+0xa/0x30 ? find_held_lock+0x72/0x90 _raw_spin_lock_irqsave+0x2f/0x50 ? swake_up_one+0xa/0x30 swake_up_one+0xa/0x30 rcutorture_one_extend+0x387/0x400 rcu_torture_one_read+0x290/0x5d0 rcu_torture_reader+0xac/0x200 ? rcutorture_oom_notify+0xf0/0xf0 ? __kthread_parkme+0x61/0x90 ? rcu_torture_one_read+0x5d0/0x5d0 kthread+0x12d/0x150 ? set_kthread_struct+0x40/0x40 ret_from_fork+0x22/0x30 This is a false positive because there is only one CPU, and both locks are raw (non-preemptible) spinlocks. However, it is worthwhile getting rid of the redundant wakeup, which has the side effect of breaking the theoretical deadlock cycle. This commit therefore eliminates the redundant wakeups. Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
272 lines
7.5 KiB
C
272 lines
7.5 KiB
C
// SPDX-License-Identifier: GPL-2.0+
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/*
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* Sleepable Read-Copy Update mechanism for mutual exclusion,
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* tiny version for non-preemptible single-CPU use.
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*
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* Copyright (C) IBM Corporation, 2017
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*
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* Author: Paul McKenney <paulmck@linux.ibm.com>
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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/preempt.h>
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#include <linux/rcupdate_wait.h>
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#include <linux/sched.h>
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#include <linux/delay.h>
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#include <linux/srcu.h>
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#include <linux/rcu_node_tree.h>
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#include "rcu_segcblist.h"
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#include "rcu.h"
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int rcu_scheduler_active __read_mostly;
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static LIST_HEAD(srcu_boot_list);
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static bool srcu_init_done;
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static int init_srcu_struct_fields(struct srcu_struct *ssp)
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{
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ssp->srcu_lock_nesting[0] = 0;
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ssp->srcu_lock_nesting[1] = 0;
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init_swait_queue_head(&ssp->srcu_wq);
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ssp->srcu_cb_head = NULL;
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ssp->srcu_cb_tail = &ssp->srcu_cb_head;
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ssp->srcu_gp_running = false;
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ssp->srcu_gp_waiting = false;
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ssp->srcu_idx = 0;
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ssp->srcu_idx_max = 0;
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INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
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INIT_LIST_HEAD(&ssp->srcu_work.entry);
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return 0;
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}
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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int __init_srcu_struct(struct srcu_struct *ssp, 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 *)ssp, sizeof(*ssp));
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lockdep_init_map(&ssp->dep_map, name, key, 0);
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return init_srcu_struct_fields(ssp);
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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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* @ssp: 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 *ssp)
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{
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return init_srcu_struct_fields(ssp);
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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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* cleanup_srcu_struct - deconstruct a sleep-RCU structure
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* @ssp: 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 *ssp)
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{
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WARN_ON(ssp->srcu_lock_nesting[0] || ssp->srcu_lock_nesting[1]);
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flush_work(&ssp->srcu_work);
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WARN_ON(ssp->srcu_gp_running);
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WARN_ON(ssp->srcu_gp_waiting);
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WARN_ON(ssp->srcu_cb_head);
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WARN_ON(&ssp->srcu_cb_head != ssp->srcu_cb_tail);
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WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
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WARN_ON(ssp->srcu_idx & 0x1);
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}
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EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
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/*
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* Removes the count for the old reader from the appropriate element of
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* the srcu_struct.
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*/
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void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
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{
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int newval = READ_ONCE(ssp->srcu_lock_nesting[idx]) - 1;
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WRITE_ONCE(ssp->srcu_lock_nesting[idx], newval);
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if (!newval && READ_ONCE(ssp->srcu_gp_waiting) && in_task())
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swake_up_one(&ssp->srcu_wq);
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}
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EXPORT_SYMBOL_GPL(__srcu_read_unlock);
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/*
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* Workqueue handler to drive one grace period and invoke any callbacks
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* that become ready as a result. Single-CPU and !PREEMPTION operation
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* means that we get away with murder on synchronization. ;-)
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*/
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void srcu_drive_gp(struct work_struct *wp)
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{
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int idx;
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struct rcu_head *lh;
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struct rcu_head *rhp;
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struct srcu_struct *ssp;
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ssp = container_of(wp, struct srcu_struct, srcu_work);
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if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
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return; /* Already running or nothing to do. */
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/* Remove recently arrived callbacks and wait for readers. */
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WRITE_ONCE(ssp->srcu_gp_running, true);
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local_irq_disable();
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lh = ssp->srcu_cb_head;
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ssp->srcu_cb_head = NULL;
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ssp->srcu_cb_tail = &ssp->srcu_cb_head;
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local_irq_enable();
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idx = (ssp->srcu_idx & 0x2) / 2;
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WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
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WRITE_ONCE(ssp->srcu_gp_waiting, true); /* srcu_read_unlock() wakes! */
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swait_event_exclusive(ssp->srcu_wq, !READ_ONCE(ssp->srcu_lock_nesting[idx]));
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WRITE_ONCE(ssp->srcu_gp_waiting, false); /* srcu_read_unlock() cheap. */
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WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
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/* Invoke the callbacks we removed above. */
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while (lh) {
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rhp = lh;
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lh = lh->next;
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local_bh_disable();
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rhp->func(rhp);
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local_bh_enable();
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}
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/*
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* Enable rescheduling, and if there are more callbacks,
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* reschedule ourselves. This can race with a call_srcu()
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* at interrupt level, but the ->srcu_gp_running checks will
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* straighten that out.
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*/
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WRITE_ONCE(ssp->srcu_gp_running, false);
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if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
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schedule_work(&ssp->srcu_work);
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}
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EXPORT_SYMBOL_GPL(srcu_drive_gp);
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static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
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{
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unsigned short cookie;
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cookie = get_state_synchronize_srcu(ssp);
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if (USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie))
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return;
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WRITE_ONCE(ssp->srcu_idx_max, cookie);
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if (!READ_ONCE(ssp->srcu_gp_running)) {
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if (likely(srcu_init_done))
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schedule_work(&ssp->srcu_work);
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else if (list_empty(&ssp->srcu_work.entry))
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list_add(&ssp->srcu_work.entry, &srcu_boot_list);
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}
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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 *ssp, struct rcu_head *rhp,
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rcu_callback_t func)
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{
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unsigned long flags;
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rhp->func = func;
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rhp->next = NULL;
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local_irq_save(flags);
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*ssp->srcu_cb_tail = rhp;
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ssp->srcu_cb_tail = &rhp->next;
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local_irq_restore(flags);
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srcu_gp_start_if_needed(ssp);
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}
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EXPORT_SYMBOL_GPL(call_srcu);
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/*
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* synchronize_srcu - wait for prior SRCU read-side critical-section completion
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*/
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void synchronize_srcu(struct srcu_struct *ssp)
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{
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struct rcu_synchronize rs;
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init_rcu_head_on_stack(&rs.head);
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init_completion(&rs.completion);
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call_srcu(ssp, &rs.head, wakeme_after_rcu);
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wait_for_completion(&rs.completion);
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destroy_rcu_head_on_stack(&rs.head);
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}
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EXPORT_SYMBOL_GPL(synchronize_srcu);
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/*
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* get_state_synchronize_srcu - Provide an end-of-grace-period cookie
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*/
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unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
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{
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unsigned long ret;
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barrier();
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ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
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barrier();
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return ret & USHRT_MAX;
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}
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EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
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/*
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* start_poll_synchronize_srcu - Provide cookie and start grace period
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*
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* The difference between this and get_state_synchronize_srcu() is that
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* this function ensures that the poll_state_synchronize_srcu() will
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* eventually return the value true.
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*/
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unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
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{
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unsigned long ret = get_state_synchronize_srcu(ssp);
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srcu_gp_start_if_needed(ssp);
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return ret;
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}
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EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
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/*
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* poll_state_synchronize_srcu - Has cookie's grace period ended?
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*/
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bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
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{
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bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
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barrier();
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return ret;
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}
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EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
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/* Lockdep diagnostics. */
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void __init rcu_scheduler_starting(void)
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{
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rcu_scheduler_active = RCU_SCHEDULER_RUNNING;
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}
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/*
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* Queue work for srcu_struct structures with early boot callbacks.
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* The work won't actually execute until the workqueue initialization
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* phase that takes place after the scheduler starts.
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*/
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void __init srcu_init(void)
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{
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struct srcu_struct *ssp;
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srcu_init_done = true;
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while (!list_empty(&srcu_boot_list)) {
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ssp = list_first_entry(&srcu_boot_list,
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struct srcu_struct, srcu_work.entry);
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list_del_init(&ssp->srcu_work.entry);
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schedule_work(&ssp->srcu_work);
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}
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}
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