Re: [PATCH v2] locking/qrwlock: Fix ordering in queued_write_lock_slowpath

[Waiman Long]

On 4/15/21 1:27 PM, 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. This exposes the window between the
> acquire and the cmpxchg to an A-B-A problem which allows reads following
> the lock acquisition to observe values speculatively before the write
> lock is truly acquired.
>
> We've seen a problem in epoll where the reader does a xchg while
> holding the read lock, but the writer can see a value change out from under it.
>
> Writer                               | Reader 2
> --------------------------------------------------------------------------------
> ep_scan_ready_list()                 |
> |- write_lock_irq()                  |
>      |- queued_write_lock_slowpath()  |
>        |- atomic_cond_read_acquire()  |
>                                       | read_lock_irqsave(&ep->lock, flags);
>     --> (observes value before unlock)|  chain_epi_lockless()
>     |                                 |    epi->next = xchg(&ep->ovflist, epi);
>     |                                 | read_unlock_irqrestore(&ep->lock, flags);
>     |                                 |
>     |     atomic_cmpxchg_relaxed()    |
>     |-- READ_ONCE(ep->ovflist);       |
>
> A core can order the read of the ovflist ahead of the
> atomic_cmpxchg_relaxed(). Switching the cmpxchg to use acquire semantics
> addresses this issue at which point the atomic_cond_read can be switched
> to use relaxed semantics.
>
> Fixes: b519b56e378ee ("locking/qrwlock: Use atomic_cond_read_acquire() when spinning in qrwlock")
> Signed-off-by: Ali Saidi <alisaidi@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Acked-by: Will Deacon <will@xxxxxxxxxx>
> Tested-by: Steve Capper <steve.capper@xxxxxxx>
> Reviewed-by: Steve Capper <steve.capper@xxxxxxx>
>
> ---
>   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);

Acked-by: Waiman Long <longman@xxxxxxxxxx>