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Commit b75a2253 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

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: default avatarPeter 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: default avatarIngo Molnar <mingo@kernel.org>
parent 2541117b

kernel/sched/core.c

+7 −1
Original line number Diff line number Diff line
@@ -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();

kernel/sched/sched.h

+3 −0
Original line number Diff line number Diff line
@@ -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);