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sched/core: Better document the try_to_wake_up() barriers

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Explain how the control dependency and smp_rmb() end up providing
ACQUIRE semantics and pair with smp_store_release() in
finish_lock_switch().

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2015-10-06 14:36:17 +02:00 committed by Ingo Molnar
parent 2541117b0c
commit b75a225315
2 changed files with 10 additions and 1 deletions
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8
kernel/sched/core.c
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@ -1953,7 +1953,13 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
while (p->on_cpu)
cpu_relax();
/*
* Pairs with the smp_wmb() in finish_lock_switch().
* Combined with the control dependency above, we have an effective
* smp_load_acquire() without the need for full barriers.
*
* Pairs with the smp_store_release() in finish_lock_switch().
*
* This ensures that tasks getting woken will be fully ordered against
* their previous state and preserve Program Order.
*/
smp_rmb();

3
kernel/sched/sched.h
View file

@ -1073,6 +1073,9 @@ static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
* We must ensure this doesn't happen until the switch is completely
* finished.
*
* In particular, the load of prev->state in finish_task_switch() must
* happen before this.
*
* Pairs with the control dependency and rmb in try_to_wake_up().
*/
smp_store_release(&prev->on_cpu, 0);