linux-kernel - [PATCH v5.2 2/2] lcoking/rwsem: Add missing ACQUIRE to read

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Date:   Mon, 26 Aug 2019 10:31:14 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     peterz@...radead.org, will@...nel.org, jstancek@...hat.com,
        stable@...r.kernel.org
Cc:     torvalds@...ux-foundation.org, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH v5.2 2/2] lcoking/rwsem: Add missing ACQUIRE to read_slowpath sleep loop

From: Peter Zijlstra <peterz@...radead.org>

[ Upstream commit 99143f82a255e7f054bead8443462fae76dd829e ]

While reviewing another read_slowpath patch, both Will and I noticed
another missing ACQUIRE, namely:

  X = 0;

  CPU0			CPU1

  rwsem_down_read()
    for (;;) {
      set_current_state(TASK_UNINTERRUPTIBLE);

                        X = 1;
                        rwsem_up_write();
                          rwsem_mark_wake()
                            atomic_long_add(adjustment, &sem->count);
                            smp_store_release(&waiter->task, NULL);

      if (!waiter.task)
        break;

      ...
    }

  r = X;

Allows 'r == 0'.

Reported-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reported-by: Will Deacon <will@...nel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Will Deacon <will@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---

This is a backport for the v5.2 stable tree. There were multiple reports  
of this issue being hit.

Given that there were a few changes to the code around this, I'd
appreciate an ack before pulling it in.

 kernel/locking/rwsem-xadd.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 397dedc58432d..385ebcfc31a6d 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -485,8 +485,10 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
 	/* wait to be given the lock */
 	while (true) {
 		set_current_state(state);
-		if (!waiter.task)
+		if (!smp_load_acquire(&waiter.task)) {
+			/* Orders against rwsem_mark_wake()'s smp_store_release() */
 			break;
+		}
 		if (signal_pending_state(state, current)) {
 			raw_spin_lock_irq(&sem->wait_lock);
 			if (waiter.task)
-- 
2.20.1