Re: [PATCH] locking/qrwlock: Fix ordering in queued_write_lock_slowpath

[Waiman Long <llong@redhat.com>]

On 4/15/21 10:25 AM, Ali Saidi wrote:
> 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);

I think the original code isn't wrong. The read_acquire provides the 
acquire barrier for cmpxchg. Because of conditional dependency, the 
wait_lock unlock won't happen until the cmpxchg succeeds. Without doing 
a read_acquire, there may be a higher likelihood that the cmpxchg may fail.

Anyway, I will let Will or Peter chime in on this as I am not as 
proficient as them on this topic.

Cheers,
Longman