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Message-ID: <tip-76e079fefc8f62bd9b2cd2950814d1ee806e31a5@git.kernel.org>
Date: Tue, 17 Jul 2018 02:30:48 -0700
From: tip-bot for Andrea Parri <tipbot@...or.com>
To: linux-tip-commits@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, peterz@...radead.org,
paulmck@...ux.vnet.ibm.com, torvalds@...ux-foundation.org,
andrea.parri@...rulasolutions.com, hpa@...or.com, mingo@...nel.org,
tglx@...utronix.de
Subject: [tip:locking/core] sched/core: Use smp_mb() in
wake_woken_function()
Commit-ID: 76e079fefc8f62bd9b2cd2950814d1ee806e31a5
Gitweb: https://git.kernel.org/tip/76e079fefc8f62bd9b2cd2950814d1ee806e31a5
Author: Andrea Parri <andrea.parri@...rulasolutions.com>
AuthorDate: Mon, 16 Jul 2018 11:06:01 -0700
Committer: Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 17 Jul 2018 09:30:33 +0200
sched/core: Use smp_mb() in wake_woken_function()
wake_woken_function() synchronizes with wait_woken() as follows:
[wait_woken] [wake_woken_function]
entry->flags &= ~wq_flag_woken; condition = true;
smp_mb(); smp_wmb();
if (condition) wq_entry->flags |= wq_flag_woken;
break;
This commit replaces the above smp_wmb() with an smp_mb() in order to
guarantee that either wait_woken() sees the wait condition being true
or the store to wq_entry->flags in woken_wake_function() follows the
store in wait_woken() in the coherence order (so that the former can
eventually be observed by wait_woken()).
The commit also fixes a comment associated to set_current_state() in
wait_woken(): the comment pairs the barrier in set_current_state() to
the above smp_wmb(), while the actual pairing involves the barrier in
set_current_state() and the barrier executed by the try_to_wake_up()
in wake_woken_function().
Signed-off-by: Andrea Parri <andrea.parri@...rulasolutions.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: akiyks@...il.com
Cc: boqun.feng@...il.com
Cc: dhowells@...hat.com
Cc: j.alglave@....ac.uk
Cc: linux-arch@...r.kernel.org
Cc: luc.maranget@...ia.fr
Cc: npiggin@...il.com
Cc: parri.andrea@...il.com
Cc: stern@...land.harvard.edu
Cc: will.deacon@....com
Link: http://lkml.kernel.org/r/20180716180605.16115-10-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
kernel/sched/wait.c | 47 +++++++++++++++++++++--------------------------
1 file changed, 21 insertions(+), 26 deletions(-)
diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
index 928be527477e..a7a2aaa3026a 100644
--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -392,35 +392,36 @@ static inline bool is_kthread_should_stop(void)
* if (condition)
* break;
*
- * p->state = mode; condition = true;
- * smp_mb(); // A smp_wmb(); // C
- * if (!wq_entry->flags & WQ_FLAG_WOKEN) wq_entry->flags |= WQ_FLAG_WOKEN;
- * schedule() try_to_wake_up();
- * p->state = TASK_RUNNING; ~~~~~~~~~~~~~~~~~~
- * wq_entry->flags &= ~WQ_FLAG_WOKEN; condition = true;
- * smp_mb() // B smp_wmb(); // C
- * wq_entry->flags |= WQ_FLAG_WOKEN;
- * }
- * remove_wait_queue(&wq_head, &wait);
+ * // in wait_woken() // in woken_wake_function()
*
+ * p->state = mode; wq_entry->flags |= WQ_FLAG_WOKEN;
+ * smp_mb(); // A try_to_wake_up():
+ * if (!(wq_entry->flags & WQ_FLAG_WOKEN)) <full barrier>
+ * schedule() if (p->state & mode)
+ * p->state = TASK_RUNNING; p->state = TASK_RUNNING;
+ * wq_entry->flags &= ~WQ_FLAG_WOKEN; ~~~~~~~~~~~~~~~~~~
+ * smp_mb(); // B condition = true;
+ * } smp_mb(); // C
+ * remove_wait_queue(&wq_head, &wait); wq_entry->flags |= WQ_FLAG_WOKEN;
*/
long wait_woken(struct wait_queue_entry *wq_entry, unsigned mode, long timeout)
{
- set_current_state(mode); /* A */
/*
- * The above implies an smp_mb(), which matches with the smp_wmb() from
- * woken_wake_function() such that if we observe WQ_FLAG_WOKEN we must
- * also observe all state before the wakeup.
+ * The below executes an smp_mb(), which matches with the full barrier
+ * executed by the try_to_wake_up() in woken_wake_function() such that
+ * either we see the store to wq_entry->flags in woken_wake_function()
+ * or woken_wake_function() sees our store to current->state.
*/
+ set_current_state(mode); /* A */
if (!(wq_entry->flags & WQ_FLAG_WOKEN) && !is_kthread_should_stop())
timeout = schedule_timeout(timeout);
__set_current_state(TASK_RUNNING);
/*
- * The below implies an smp_mb(), it too pairs with the smp_wmb() from
- * woken_wake_function() such that we must either observe the wait
- * condition being true _OR_ WQ_FLAG_WOKEN such that we will not miss
- * an event.
+ * The below executes an smp_mb(), which matches with the smp_mb() (C)
+ * in woken_wake_function() such that either we see the wait condition
+ * being true or the store to wq_entry->flags in woken_wake_function()
+ * follows ours in the coherence order.
*/
smp_store_mb(wq_entry->flags, wq_entry->flags & ~WQ_FLAG_WOKEN); /* B */
@@ -430,14 +431,8 @@ EXPORT_SYMBOL(wait_woken);
int woken_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int sync, void *key)
{
- /*
- * Although this function is called under waitqueue lock, LOCK
- * doesn't imply write barrier and the users expects write
- * barrier semantics on wakeup functions. The following
- * smp_wmb() is equivalent to smp_wmb() in try_to_wake_up()
- * and is paired with smp_store_mb() in wait_woken().
- */
- smp_wmb(); /* C */
+ /* Pairs with the smp_store_mb() in wait_woken(). */
+ smp_mb(); /* C */
wq_entry->flags |= WQ_FLAG_WOKEN;
return default_wake_function(wq_entry, mode, sync, key);
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