linux-kernel - [PATCH 4/4] rwsem: Add ACQUIRE comments

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Date:   Thu, 18 Jul 2019 15:49:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Jan Stancek <jstancek@...hat.com>,
        Waiman Long <longman@...hat.com>, dbueso@...e.de,
        mingo@...hat.com, jade.alglave@....com, paulmck@...ux.vnet.ibm.com
Subject: [PATCH 4/4] rwsem: Add ACQUIRE comments

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

Acked-by: Will Deacon <will@...nel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/locking/rwsem.c |   18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -1000,6 +1000,7 @@ rwsem_down_read_slowpath(struct rw_semap
 	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.
@@ -1014,6 +1015,7 @@ rwsem_down_read_slowpath(struct rw_semap
 		}
 		return sem;
 	} else if (rwsem_reader_phase_trylock(sem, waiter.last_rowner)) {
+		/* rwsem_reader_phase_trylock() implies ACQUIRE on success */
 		return sem;
 	}
 
@@ -1067,10 +1069,10 @@ rwsem_down_read_slowpath(struct rw_semap
 	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)) {
@@ -1078,6 +1080,7 @@ rwsem_down_read_slowpath(struct rw_semap
 			if (waiter.task)
 				goto out_nolock;
 			raw_spin_unlock_irq(&sem->wait_lock);
+			/* Ordered by sem->wait_lock against rwsem_mark_wake(). */
 			break;
 		}
 		schedule();
@@ -1087,6 +1090,7 @@ rwsem_down_read_slowpath(struct rw_semap
 	__set_current_state(TASK_RUNNING);
 	lockevent_inc(rwsem_rlock);
 	return sem;
+
 out_nolock:
 	list_del(&waiter.list);
 	if (list_empty(&sem->wait_list)) {
@@ -1127,8 +1131,10 @@ rwsem_down_write_slowpath(struct rw_sema
 
 	/* 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
@@ -1188,9 +1194,11 @@ rwsem_down_write_slowpath(struct rw_sema
 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);