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

[Will Deacon <will@kernel.org>]

On Thu, Apr 15, 2021 at 02:25:52PM +0000, 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.

I think it would be worth spelling this out with an example. The issue
appears to be a concurrent reader in interrupt context taking and releasing
the lock in the window where the writer has returned from the
atomic_cond_read_acquire() but has not yet performed the cmpxchg(). Loads
can be speculated during this time, but the A-B-A of the lock word
from _QW_WAITING to (_QW_WAITING | _QR_BIAS) and back to _QW_WAITING allows
the atomic_cmpxchg_relaxed() to succeed. Is that right?

With that in mind, it would probably be a good idea to eyeball the qspinlock
slowpath as well, as that uses both atomic_cond_read_acquire() and
atomic_try_cmpxchg_relaxed().

> Fixes: b519b56e378ee ("locking/qrwlock: Use atomic_cond_read_acquire() when spinning in qrwloc")

Typo in the quoted subject ('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);

Patch looks good, so with an updated message:

Acked-by: Will Deacon <will@kernel.org>

Will