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From:   Ali Saidi <alisaidi@amazon.com>
To:     <linux-kernel@vger.kernel.org>
CC:     <alisaidi@amazon.com>, <catalin.marinas@arm.com>,
        <steve.capper@arm.com>, <benh@kernel.crashing.org>,
        <stable@vger.kernel.org>, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>
Subject: [PATCH] locking/qrwlock: Fix ordering in queued_write_lock_slowpath
Date:   Thu, 15 Apr 2021 14:25:52 +0000
Message-ID: <20210415142552.30916-1-alisaidi@amazon.com>
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Precedence: bulk

While this code is executed with the wait_lock held, a reader can
acquire the lock without holding wait_lock.  The writer side loops
checking the value with the atomic_cond_read_acquire(), but only truly
acquires the lock when the compare-and-exchange is completed
successfully which isn’t ordered. The other atomic operations from this
point are release-ordered and thus reads after the lock acquisition can
be completed before the lock is truly acquired which violates the
guarantees the lock should be making.

Fixes: b519b56e378ee ("locking/qrwlock: Use atomic_cond_read_acquire() when spinning in qrwloc")
Signed-off-by: Ali Saidi <alisaidi@amazon.com>
Cc: stable@vger.kernel.org
---
 kernel/locking/qrwlock.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index 4786dd271b45..10770f6ac4d9 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -73,8 +73,8 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
 
 	/* When no more readers or writers, set the locked flag */
 	do {
-		atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
-	} while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING,
+		atomic_cond_read_relaxed(&lock->cnts, VAL == _QW_WAITING);
+	} while (atomic_cmpxchg_acquire(&lock->cnts, _QW_WAITING,
 					_QW_LOCKED) != _QW_WAITING);
 unlock:
 	arch_spin_unlock(&lock->wait_lock);
-- 
2.24.4.AMZN