* [PATCH 4.18 166/235] sched/core: Use smp_mb() in wake_woken_function() [not found] <20180924113103.999624566@linuxfoundation.org> @ 2018-09-24 11:52 ` Greg Kroah-Hartman 2018-09-24 11:52 ` Greg Kroah-Hartman 0 siblings, 1 reply; 2+ messages in thread From: Greg Kroah-Hartman @ 2018-09-24 11:52 UTC (permalink / raw) To: linux-kernel Cc: Greg Kroah-Hartman, stable, Andrea Parri, Paul E. McKenney, Peter Zijlstra (Intel), Linus Torvalds, Thomas Gleixner, akiyks, boqun.feng, dhowells, j.alglave, linux-arch, luc.maranget, npiggin, parri.andrea, stern, will.deacon, Ingo Molnar, Sasha Levin 4.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Andrea Parri <andrea.parri@amarulasolutions.com> [ Upstream commit 76e079fefc8f62bd9b2cd2950814d1ee806e31a5 ] 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@amarulasolutions.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: akiyks@gmail.com Cc: boqun.feng@gmail.com Cc: dhowells@redhat.com Cc: j.alglave@ucl.ac.uk Cc: linux-arch@vger.kernel.org Cc: luc.maranget@inria.fr Cc: npiggin@gmail.com Cc: parri.andrea@gmail.com Cc: stern@rowland.harvard.edu Cc: will.deacon@arm.com Link: http://lkml.kernel.org/r/20180716180605.16115-10-paulmck@linux.vnet.ibm.com Signed-off-by: Ingo Molnar <mingo@kernel.org> Signed-off-by: Sasha Levin <alexander.levin@microsoft.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> --- kernel/sched/wait.c | 47 +++++++++++++++++++++-------------------------- 1 file changed, 21 insertions(+), 26 deletions(-) --- a/kernel/sched/wait.c +++ b/kernel/sched/wait.c @@ -392,35 +392,36 @@ static inline bool is_kthread_should_sto * 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); ^ permalink raw reply [flat|nested] 2+ messages in thread
* [PATCH 4.18 166/235] sched/core: Use smp_mb() in wake_woken_function() 2018-09-24 11:52 ` [PATCH 4.18 166/235] sched/core: Use smp_mb() in wake_woken_function() Greg Kroah-Hartman @ 2018-09-24 11:52 ` Greg Kroah-Hartman 0 siblings, 0 replies; 2+ messages in thread From: Greg Kroah-Hartman @ 2018-09-24 11:52 UTC (permalink / raw) To: linux-kernel Cc: Greg Kroah-Hartman, stable, Andrea Parri, Paul E. McKenney, Peter Zijlstra (Intel), Linus Torvalds, Thomas Gleixner, akiyks, boqun.feng, dhowells, j.alglave, linux-arch, luc.maranget, npiggin, parri.andrea, stern, will.deacon, Ingo Molnar, Sasha Levin 4.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Andrea Parri <andrea.parri@amarulasolutions.com> [ Upstream commit 76e079fefc8f62bd9b2cd2950814d1ee806e31a5 ] 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@amarulasolutions.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: akiyks@gmail.com Cc: boqun.feng@gmail.com Cc: dhowells@redhat.com Cc: j.alglave@ucl.ac.uk Cc: linux-arch@vger.kernel.org Cc: luc.maranget@inria.fr Cc: npiggin@gmail.com Cc: parri.andrea@gmail.com Cc: stern@rowland.harvard.edu Cc: will.deacon@arm.com Link: http://lkml.kernel.org/r/20180716180605.16115-10-paulmck@linux.vnet.ibm.com Signed-off-by: Ingo Molnar <mingo@kernel.org> Signed-off-by: Sasha Levin <alexander.levin@microsoft.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> --- kernel/sched/wait.c | 47 +++++++++++++++++++++-------------------------- 1 file changed, 21 insertions(+), 26 deletions(-) --- a/kernel/sched/wait.c +++ b/kernel/sched/wait.c @@ -392,35 +392,36 @@ static inline bool is_kthread_should_sto * 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); ^ permalink raw reply [flat|nested] 2+ messages in thread
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2018-09-24 11:52 ` [PATCH 4.18 166/235] sched/core: Use smp_mb() in wake_woken_function() Greg Kroah-Hartman
2018-09-24 11:52 ` Greg Kroah-Hartman
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