sched/core: Better document the try_to_wake_up() barriers

Peter Zijlstra · Ingo Molnar · commit b75a22531588 · 2015-12-04T10:26:42.000+01:00
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) &lt;peterz@infradead.org&gt;
Cc: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
Cc: Mike Galbraith &lt;efault@gmx.de&gt;
Cc: Oleg Nesterov &lt;oleg@redhat.com&gt;
Cc: Paul E. McKenney &lt;paulmck@linux.vnet.ibm.com&gt;
Cc: Peter Zijlstra &lt;peterz@infradead.org&gt;
Cc: Thomas Gleixner &lt;tglx@linutronix.de&gt;
Signed-off-by: Ingo Molnar &lt;mingo@kernel.org&gt;
diff --git 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
@@ -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);
