Commit-ID: b75a22531588e77aa8c2daf228c9723916ae2cd0 Gitweb: http://git.kernel.org/tip/b75a22531588e77aa8c2daf228c9723916ae2cd0 Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx> AuthorDate: Tue, 6 Oct 2015 14:36:17 +0200 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitDate: Fri, 4 Dec 2015 10:26:42 +0100 sched/core: Better document the try_to_wake_up() barriers 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@xxxxxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Mike Galbraith <efault@xxxxxx> Cc: Oleg Nesterov <oleg@xxxxxxxxxx> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> --- kernel/sched/core.c | 8 +++++++- kernel/sched/sched.h | 3 +++ 2 files changed, 10 insertions(+), 1 deletion(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index eee4ee6..b64f163 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -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(); diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index efd3bfc..b242775 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -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); -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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