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![Davidlohr Bueso](/assets/img/avatar_default.png)
When wanting to wakeup readers, __rwsem_mark_wakeup() currently iterates the wait_list twice while looking to wakeup the first N queued reader-tasks. While this can be quite inefficient, it was there such that a awoken reader would be first and foremost acknowledged by the lock counter. Keeping the same logic, we can further benefit from the use of wake_qs and avoid entirely the first wait_list iteration that sets the counter as wake_up_process() isn't going to occur right away, and therefore we maintain the counter->list order of going about things. Other than saving cycles with O(n) "scanning", this change also nicely cleans up a good chunk of __rwsem_mark_wakeup(); both visually and less tedious to read. For example, the following improvements where seen on some will it scale microbenchmarks, on a 48-core Haswell: v4.7 v4.7-rwsem-v1 Hmean signal1-processes-8 5792691.42 ( 0.00%) 5771971.04 ( -0.36%) Hmean signal1-processes-12 6081199.96 ( 0.00%) 6072174.38 ( -0.15%) Hmean signal1-processes-21 3071137.71 ( 0.00%) 3041336.72 ( -0.97%) Hmean signal1-processes-48 3712039.98 ( 0.00%) 3708113.59 ( -0.11%) Hmean signal1-processes-79 4464573.45 ( 0.00%) 4682798.66 ( 4.89%) Hmean signal1-processes-110 4486842.01 ( 0.00%) 4633781.71 ( 3.27%) Hmean signal1-processes-141 4611816.83 ( 0.00%) 4692725.38 ( 1.75%) Hmean signal1-processes-172 4638157.05 ( 0.00%) 4714387.86 ( 1.64%) Hmean signal1-processes-203 4465077.80 ( 0.00%) 4690348.07 ( 5.05%) Hmean signal1-processes-224 4410433.74 ( 0.00%) 4687534.43 ( 6.28%) Stddev signal1-processes-8 6360.47 ( 0.00%) 8455.31 ( 32.94%) Stddev signal1-processes-12 4004.98 ( 0.00%) 9156.13 (128.62%) Stddev signal1-processes-21 3273.14 ( 0.00%) 5016.80 ( 53.27%) Stddev signal1-processes-48 28420.25 ( 0.00%) 26576.22 ( -6.49%) Stddev signal1-processes-79 22038.34 ( 0.00%) 18992.70 (-13.82%) Stddev signal1-processes-110 23226.93 ( 0.00%) 17245.79 (-25.75%) Stddev signal1-processes-141 6358.98 ( 0.00%) 7636.14 ( 20.08%) Stddev signal1-processes-172 9523.70 ( 0.00%) 4824.75 (-49.34%) Stddev signal1-processes-203 13915.33 ( 0.00%) 9326.33 (-32.98%) Stddev signal1-processes-224 15573.94 ( 0.00%) 10613.82 (-31.85%) Other runs that saw improvements include context_switch and pipe; and as expected, this is particularly highlighted on larger thread counts as it becomes more expensive to walk the list twice. No change in wakeup ordering or semantics. Signed-off-by: Davidlohr Bueso <dbueso@suse.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Waiman.Long@hp.com Cc: dave@stgolabs.net Cc: jason.low2@hpe.com Cc: wanpeng.li@hotmail.com Link: http://lkml.kernel.org/r/1470384285-32163-4-git-send-email-dave@stgolabs.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
640 lines
18 KiB
C
640 lines
18 KiB
C
/* rwsem.c: R/W semaphores: contention handling functions
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*
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* Written by David Howells (dhowells@redhat.com).
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* Derived from arch/i386/kernel/semaphore.c
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*
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* Writer lock-stealing by Alex Shi <alex.shi@intel.com>
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* and Michel Lespinasse <walken@google.com>
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*
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* Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
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* and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
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*/
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#include <linux/rwsem.h>
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#include <linux/sched.h>
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#include <linux/init.h>
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#include <linux/export.h>
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#include <linux/sched/rt.h>
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#include <linux/osq_lock.h>
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#include "rwsem.h"
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/*
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* Guide to the rw_semaphore's count field for common values.
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* (32-bit case illustrated, similar for 64-bit)
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*
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* 0x0000000X (1) X readers active or attempting lock, no writer waiting
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* X = #active_readers + #readers attempting to lock
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* (X*ACTIVE_BIAS)
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*
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* 0x00000000 rwsem is unlocked, and no one is waiting for the lock or
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* attempting to read lock or write lock.
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*
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* 0xffff000X (1) X readers active or attempting lock, with waiters for lock
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* X = #active readers + # readers attempting lock
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* (X*ACTIVE_BIAS + WAITING_BIAS)
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* (2) 1 writer attempting lock, no waiters for lock
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* X-1 = #active readers + #readers attempting lock
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* ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
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* (3) 1 writer active, no waiters for lock
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* X-1 = #active readers + #readers attempting lock
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* ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
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*
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* 0xffff0001 (1) 1 reader active or attempting lock, waiters for lock
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* (WAITING_BIAS + ACTIVE_BIAS)
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* (2) 1 writer active or attempting lock, no waiters for lock
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* (ACTIVE_WRITE_BIAS)
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*
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* 0xffff0000 (1) There are writers or readers queued but none active
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* or in the process of attempting lock.
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* (WAITING_BIAS)
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* Note: writer can attempt to steal lock for this count by adding
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* ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
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*
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* 0xfffe0001 (1) 1 writer active, or attempting lock. Waiters on queue.
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* (ACTIVE_WRITE_BIAS + WAITING_BIAS)
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*
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* Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
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* the count becomes more than 0 for successful lock acquisition,
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* i.e. the case where there are only readers or nobody has lock.
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* (1st and 2nd case above).
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*
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* Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
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* checking the count becomes ACTIVE_WRITE_BIAS for successful lock
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* acquisition (i.e. nobody else has lock or attempts lock). If
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* unsuccessful, in rwsem_down_write_failed, we'll check to see if there
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* are only waiters but none active (5th case above), and attempt to
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* steal the lock.
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*
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*/
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/*
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* Initialize an rwsem:
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*/
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void __init_rwsem(struct rw_semaphore *sem, const char *name,
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struct lock_class_key *key)
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{
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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/*
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* Make sure we are not reinitializing a held semaphore:
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*/
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debug_check_no_locks_freed((void *)sem, sizeof(*sem));
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lockdep_init_map(&sem->dep_map, name, key, 0);
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#endif
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atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
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raw_spin_lock_init(&sem->wait_lock);
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INIT_LIST_HEAD(&sem->wait_list);
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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sem->owner = NULL;
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osq_lock_init(&sem->osq);
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#endif
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}
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EXPORT_SYMBOL(__init_rwsem);
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enum rwsem_waiter_type {
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RWSEM_WAITING_FOR_WRITE,
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RWSEM_WAITING_FOR_READ
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};
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struct rwsem_waiter {
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struct list_head list;
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struct task_struct *task;
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enum rwsem_waiter_type type;
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};
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enum rwsem_wake_type {
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RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
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RWSEM_WAKE_READERS, /* Wake readers only */
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RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
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};
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/*
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* handle the lock release when processes blocked on it that can now run
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* - if we come here from up_xxxx(), then:
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* - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
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* - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
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* - there must be someone on the queue
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* - the wait_lock must be held by the caller
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* - tasks are marked for wakeup, the caller must later invoke wake_up_q()
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* to actually wakeup the blocked task(s) and drop the reference count,
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* preferably when the wait_lock is released
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* - woken process blocks are discarded from the list after having task zeroed
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* - writers are only marked woken if downgrading is false
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*/
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static void __rwsem_mark_wake(struct rw_semaphore *sem,
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enum rwsem_wake_type wake_type,
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struct wake_q_head *wake_q)
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{
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struct rwsem_waiter *waiter, *tmp;
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long oldcount, woken = 0, adjustment = 0;
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/*
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* Take a peek at the queue head waiter such that we can determine
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* the wakeup(s) to perform.
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*/
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waiter = list_first_entry(&sem->wait_list, struct rwsem_waiter, list);
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if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
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if (wake_type == RWSEM_WAKE_ANY) {
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/*
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* Mark writer at the front of the queue for wakeup.
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* Until the task is actually later awoken later by
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* the caller, other writers are able to steal it.
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* Readers, on the other hand, will block as they
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* will notice the queued writer.
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*/
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wake_q_add(wake_q, waiter->task);
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}
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return;
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}
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/*
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* Writers might steal the lock before we grant it to the next reader.
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* We prefer to do the first reader grant before counting readers
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* so we can bail out early if a writer stole the lock.
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*/
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if (wake_type != RWSEM_WAKE_READ_OWNED) {
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adjustment = RWSEM_ACTIVE_READ_BIAS;
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try_reader_grant:
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oldcount = atomic_long_fetch_add(adjustment, &sem->count);
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if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
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/*
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* If the count is still less than RWSEM_WAITING_BIAS
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* after removing the adjustment, it is assumed that
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* a writer has stolen the lock. We have to undo our
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* reader grant.
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*/
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if (atomic_long_add_return(-adjustment, &sem->count) <
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RWSEM_WAITING_BIAS)
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return;
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/* Last active locker left. Retry waking readers. */
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goto try_reader_grant;
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}
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/*
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* It is not really necessary to set it to reader-owned here,
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* but it gives the spinners an early indication that the
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* readers now have the lock.
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*/
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rwsem_set_reader_owned(sem);
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}
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/*
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* Grant an infinite number of read locks to the readers at the front
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* of the queue. We know that woken will be at least 1 as we accounted
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* for above. Note we increment the 'active part' of the count by the
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* number of readers before waking any processes up.
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*/
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list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
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struct task_struct *tsk;
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if (waiter->type == RWSEM_WAITING_FOR_WRITE)
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break;
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woken++;
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tsk = waiter->task;
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wake_q_add(wake_q, tsk);
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list_del(&waiter->list);
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/*
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* Ensure that the last operation is setting the reader
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* waiter to nil such that rwsem_down_read_failed() cannot
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* race with do_exit() by always holding a reference count
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* to the task to wakeup.
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*/
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smp_store_release(&waiter->task, NULL);
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}
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adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
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if (list_empty(&sem->wait_list)) {
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/* hit end of list above */
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adjustment -= RWSEM_WAITING_BIAS;
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}
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if (adjustment)
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atomic_long_add(adjustment, &sem->count);
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}
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/*
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* Wait for the read lock to be granted
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*/
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__visible
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struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
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{
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long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
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struct rwsem_waiter waiter;
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struct task_struct *tsk = current;
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WAKE_Q(wake_q);
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waiter.task = tsk;
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waiter.type = RWSEM_WAITING_FOR_READ;
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raw_spin_lock_irq(&sem->wait_lock);
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if (list_empty(&sem->wait_list))
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adjustment += RWSEM_WAITING_BIAS;
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list_add_tail(&waiter.list, &sem->wait_list);
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/* we're now waiting on the lock, but no longer actively locking */
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count = atomic_long_add_return(adjustment, &sem->count);
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/*
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* If there are no active locks, wake the front queued process(es).
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*
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* If there are no writers and we are first in the queue,
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* wake our own waiter to join the existing active readers !
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*/
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if (count == RWSEM_WAITING_BIAS ||
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(count > RWSEM_WAITING_BIAS &&
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adjustment != -RWSEM_ACTIVE_READ_BIAS))
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__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
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raw_spin_unlock_irq(&sem->wait_lock);
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wake_up_q(&wake_q);
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/* wait to be given the lock */
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while (true) {
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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if (!waiter.task)
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break;
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schedule();
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}
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__set_task_state(tsk, TASK_RUNNING);
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return sem;
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}
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EXPORT_SYMBOL(rwsem_down_read_failed);
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/*
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* This function must be called with the sem->wait_lock held to prevent
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* race conditions between checking the rwsem wait list and setting the
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* sem->count accordingly.
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*/
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static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
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{
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/*
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* Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS.
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*/
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if (count != RWSEM_WAITING_BIAS)
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return false;
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/*
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* Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there
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* are other tasks on the wait list, we need to add on WAITING_BIAS.
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*/
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count = list_is_singular(&sem->wait_list) ?
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RWSEM_ACTIVE_WRITE_BIAS :
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RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS;
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if (atomic_long_cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count)
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== RWSEM_WAITING_BIAS) {
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rwsem_set_owner(sem);
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return true;
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}
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return false;
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}
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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/*
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* Try to acquire write lock before the writer has been put on wait queue.
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*/
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static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
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{
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long old, count = atomic_long_read(&sem->count);
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while (true) {
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if (!(count == 0 || count == RWSEM_WAITING_BIAS))
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return false;
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old = atomic_long_cmpxchg_acquire(&sem->count, count,
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count + RWSEM_ACTIVE_WRITE_BIAS);
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if (old == count) {
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rwsem_set_owner(sem);
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return true;
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}
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count = old;
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}
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}
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static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
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{
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struct task_struct *owner;
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bool ret = true;
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if (need_resched())
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return false;
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rcu_read_lock();
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owner = READ_ONCE(sem->owner);
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if (!rwsem_owner_is_writer(owner)) {
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/*
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* Don't spin if the rwsem is readers owned.
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*/
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ret = !rwsem_owner_is_reader(owner);
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goto done;
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}
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ret = owner->on_cpu;
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done:
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rcu_read_unlock();
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return ret;
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}
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/*
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* Return true only if we can still spin on the owner field of the rwsem.
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*/
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static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
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{
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struct task_struct *owner = READ_ONCE(sem->owner);
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if (!rwsem_owner_is_writer(owner))
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goto out;
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rcu_read_lock();
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while (sem->owner == owner) {
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/*
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* Ensure we emit the owner->on_cpu, dereference _after_
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* checking sem->owner still matches owner, if that fails,
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* owner might point to free()d memory, if it still matches,
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* the rcu_read_lock() ensures the memory stays valid.
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*/
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barrier();
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/* abort spinning when need_resched or owner is not running */
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if (!owner->on_cpu || need_resched()) {
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rcu_read_unlock();
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return false;
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}
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cpu_relax_lowlatency();
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}
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rcu_read_unlock();
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out:
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/*
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* If there is a new owner or the owner is not set, we continue
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* spinning.
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*/
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return !rwsem_owner_is_reader(READ_ONCE(sem->owner));
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}
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static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
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{
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bool taken = false;
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preempt_disable();
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/* sem->wait_lock should not be held when doing optimistic spinning */
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if (!rwsem_can_spin_on_owner(sem))
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goto done;
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if (!osq_lock(&sem->osq))
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goto done;
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/*
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* Optimistically spin on the owner field and attempt to acquire the
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* lock whenever the owner changes. Spinning will be stopped when:
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* 1) the owning writer isn't running; or
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* 2) readers own the lock as we can't determine if they are
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* actively running or not.
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*/
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while (rwsem_spin_on_owner(sem)) {
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/*
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* Try to acquire the lock
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*/
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if (rwsem_try_write_lock_unqueued(sem)) {
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taken = true;
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break;
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}
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/*
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* When there's no owner, we might have preempted between the
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* owner acquiring the lock and setting the owner field. If
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* we're an RT task that will live-lock because we won't let
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* the owner complete.
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*/
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if (!sem->owner && (need_resched() || rt_task(current)))
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break;
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/*
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* The cpu_relax() call is a compiler barrier which forces
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* everything in this loop to be re-loaded. We don't need
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* memory barriers as we'll eventually observe the right
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* values at the cost of a few extra spins.
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*/
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cpu_relax_lowlatency();
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}
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osq_unlock(&sem->osq);
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done:
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preempt_enable();
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return taken;
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}
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/*
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* Return true if the rwsem has active spinner
|
|
*/
|
|
static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
|
|
{
|
|
return osq_is_locked(&sem->osq);
|
|
}
|
|
|
|
#else
|
|
static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
|
|
{
|
|
return false;
|
|
}
|
|
|
|
static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
|
|
{
|
|
return false;
|
|
}
|
|
#endif
|
|
|
|
/*
|
|
* Wait until we successfully acquire the write lock
|
|
*/
|
|
static inline struct rw_semaphore *
|
|
__rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
|
|
{
|
|
long count;
|
|
bool waiting = true; /* any queued threads before us */
|
|
struct rwsem_waiter waiter;
|
|
struct rw_semaphore *ret = sem;
|
|
WAKE_Q(wake_q);
|
|
|
|
/* undo write bias from down_write operation, stop active locking */
|
|
count = atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS, &sem->count);
|
|
|
|
/* do optimistic spinning and steal lock if possible */
|
|
if (rwsem_optimistic_spin(sem))
|
|
return sem;
|
|
|
|
/*
|
|
* Optimistic spinning failed, proceed to the slowpath
|
|
* and block until we can acquire the sem.
|
|
*/
|
|
waiter.task = current;
|
|
waiter.type = RWSEM_WAITING_FOR_WRITE;
|
|
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
|
|
/* account for this before adding a new element to the list */
|
|
if (list_empty(&sem->wait_list))
|
|
waiting = false;
|
|
|
|
list_add_tail(&waiter.list, &sem->wait_list);
|
|
|
|
/* we're now waiting on the lock, but no longer actively locking */
|
|
if (waiting) {
|
|
count = atomic_long_read(&sem->count);
|
|
|
|
/*
|
|
* If there were already threads queued before us and there are
|
|
* no active writers, the lock must be read owned; so we try to
|
|
* wake any read locks that were queued ahead of us.
|
|
*/
|
|
if (count > RWSEM_WAITING_BIAS) {
|
|
WAKE_Q(wake_q);
|
|
|
|
__rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q);
|
|
/*
|
|
* The wakeup is normally called _after_ the wait_lock
|
|
* is released, but given that we are proactively waking
|
|
* readers we can deal with the wake_q overhead as it is
|
|
* similar to releasing and taking the wait_lock again
|
|
* for attempting rwsem_try_write_lock().
|
|
*/
|
|
wake_up_q(&wake_q);
|
|
}
|
|
|
|
} else
|
|
count = atomic_long_add_return(RWSEM_WAITING_BIAS, &sem->count);
|
|
|
|
/* wait until we successfully acquire the lock */
|
|
set_current_state(state);
|
|
while (true) {
|
|
if (rwsem_try_write_lock(count, sem))
|
|
break;
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
|
|
/* Block until there are no active lockers. */
|
|
do {
|
|
if (signal_pending_state(state, current))
|
|
goto out_nolock;
|
|
|
|
schedule();
|
|
set_current_state(state);
|
|
} while ((count = atomic_long_read(&sem->count)) & RWSEM_ACTIVE_MASK);
|
|
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
}
|
|
__set_current_state(TASK_RUNNING);
|
|
list_del(&waiter.list);
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
|
|
return ret;
|
|
|
|
out_nolock:
|
|
__set_current_state(TASK_RUNNING);
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
list_del(&waiter.list);
|
|
if (list_empty(&sem->wait_list))
|
|
atomic_long_add(-RWSEM_WAITING_BIAS, &sem->count);
|
|
else
|
|
__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
wake_up_q(&wake_q);
|
|
|
|
return ERR_PTR(-EINTR);
|
|
}
|
|
|
|
__visible struct rw_semaphore * __sched
|
|
rwsem_down_write_failed(struct rw_semaphore *sem)
|
|
{
|
|
return __rwsem_down_write_failed_common(sem, TASK_UNINTERRUPTIBLE);
|
|
}
|
|
EXPORT_SYMBOL(rwsem_down_write_failed);
|
|
|
|
__visible struct rw_semaphore * __sched
|
|
rwsem_down_write_failed_killable(struct rw_semaphore *sem)
|
|
{
|
|
return __rwsem_down_write_failed_common(sem, TASK_KILLABLE);
|
|
}
|
|
EXPORT_SYMBOL(rwsem_down_write_failed_killable);
|
|
|
|
/*
|
|
* handle waking up a waiter on the semaphore
|
|
* - up_read/up_write has decremented the active part of count if we come here
|
|
*/
|
|
__visible
|
|
struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
|
|
{
|
|
unsigned long flags;
|
|
WAKE_Q(wake_q);
|
|
|
|
/*
|
|
* If a spinner is present, it is not necessary to do the wakeup.
|
|
* Try to do wakeup only if the trylock succeeds to minimize
|
|
* spinlock contention which may introduce too much delay in the
|
|
* unlock operation.
|
|
*
|
|
* spinning writer up_write/up_read caller
|
|
* --------------- -----------------------
|
|
* [S] osq_unlock() [L] osq
|
|
* MB RMB
|
|
* [RmW] rwsem_try_write_lock() [RmW] spin_trylock(wait_lock)
|
|
*
|
|
* Here, it is important to make sure that there won't be a missed
|
|
* wakeup while the rwsem is free and the only spinning writer goes
|
|
* to sleep without taking the rwsem. Even when the spinning writer
|
|
* is just going to break out of the waiting loop, it will still do
|
|
* a trylock in rwsem_down_write_failed() before sleeping. IOW, if
|
|
* rwsem_has_spinner() is true, it will guarantee at least one
|
|
* trylock attempt on the rwsem later on.
|
|
*/
|
|
if (rwsem_has_spinner(sem)) {
|
|
/*
|
|
* The smp_rmb() here is to make sure that the spinner
|
|
* state is consulted before reading the wait_lock.
|
|
*/
|
|
smp_rmb();
|
|
if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
|
|
return sem;
|
|
goto locked;
|
|
}
|
|
raw_spin_lock_irqsave(&sem->wait_lock, flags);
|
|
locked:
|
|
|
|
if (!list_empty(&sem->wait_list))
|
|
__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
|
|
|
|
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
|
|
wake_up_q(&wake_q);
|
|
|
|
return sem;
|
|
}
|
|
EXPORT_SYMBOL(rwsem_wake);
|
|
|
|
/*
|
|
* downgrade a write lock into a read lock
|
|
* - caller incremented waiting part of count and discovered it still negative
|
|
* - just wake up any readers at the front of the queue
|
|
*/
|
|
__visible
|
|
struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
|
|
{
|
|
unsigned long flags;
|
|
WAKE_Q(wake_q);
|
|
|
|
raw_spin_lock_irqsave(&sem->wait_lock, flags);
|
|
|
|
if (!list_empty(&sem->wait_list))
|
|
__rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
|
|
|
|
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
|
|
wake_up_q(&wake_q);
|
|
|
|
return sem;
|
|
}
|
|
EXPORT_SYMBOL(rwsem_downgrade_wake);
|