[tip:locking/core] locking/rwsem: Add ACQUIRE comments

[tip-bot for Peter Zijlstra <tipbot@xxxxxxxxx>]

Commit-ID:  6ffddfb9e1de21c3d0c0cfa4fe4a20dd3291a812
Gitweb:     https://git.kernel.org/tip/6ffddfb9e1de21c3d0c0cfa4fe4a20dd3291a812
Author:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Thu, 18 Jul 2019 15:08:53 +0200
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Thu, 25 Jul 2019 15:39:25 +0200

locking/rwsem: Add ACQUIRE comments

Since we just reviewed read_slowpath for ACQUIRE correctness, add a
few coments to retain our findings.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Will Deacon <will@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/locking/rwsem.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 571938887cc8..bd0f0d05724c 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -1004,6 +1004,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state)
 	atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
 	adjustment = 0;
 	if (rwsem_optimistic_spin(sem, false)) {
+		/* rwsem_optimistic_spin() implies ACQUIRE on success */
 		/*
 		 * Wake up other readers in the wait list if the front
 		 * waiter is a reader.
@@ -1018,6 +1019,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state)
 		}
 		return sem;
 	} else if (rwsem_reader_phase_trylock(sem, waiter.last_rowner)) {
+		/* rwsem_reader_phase_trylock() implies ACQUIRE on success */
 		return sem;
 	}
 
@@ -1071,10 +1073,10 @@ queue:
 	wake_up_q(&wake_q);
 
 	/* wait to be given the lock */
-	while (true) {
+	for (;;) {
 		set_current_state(state);
 		if (!smp_load_acquire(&waiter.task)) {
-			/* Orders against rwsem_mark_wake()'s smp_store_release() */
+			/* Matches rwsem_mark_wake()'s smp_store_release(). */
 			break;
 		}
 		if (signal_pending_state(state, current)) {
@@ -1082,6 +1084,7 @@ queue:
 			if (waiter.task)
 				goto out_nolock;
 			raw_spin_unlock_irq(&sem->wait_lock);
+			/* Ordered by sem->wait_lock against rwsem_mark_wake(). */
 			break;
 		}
 		schedule();
@@ -1091,6 +1094,7 @@ queue:
 	__set_current_state(TASK_RUNNING);
 	lockevent_inc(rwsem_rlock);
 	return sem;
+
 out_nolock:
 	list_del(&waiter.list);
 	if (list_empty(&sem->wait_list)) {
@@ -1131,8 +1135,10 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
 
 	/* do optimistic spinning and steal lock if possible */
 	if (rwsem_can_spin_on_owner(sem, RWSEM_WR_NONSPINNABLE) &&
-	    rwsem_optimistic_spin(sem, true))
+	    rwsem_optimistic_spin(sem, true)) {
+		/* rwsem_optimistic_spin() implies ACQUIRE on success */
 		return sem;
+	}
 
 	/*
 	 * Disable reader optimistic spinning for this rwsem after
@@ -1192,9 +1198,11 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
 wait:
 	/* wait until we successfully acquire the lock */
 	set_current_state(state);
-	while (true) {
-		if (rwsem_try_write_lock(sem, wstate))
+	for (;;) {
+		if (rwsem_try_write_lock(sem, wstate)) {
+			/* rwsem_try_write_lock() implies ACQUIRE on success */
 			break;
+		}
 
 		raw_spin_unlock_irq(&sem->wait_lock);