Re: [PATCH] locking/rwsem: Synchronize task state & waiter->task of readers

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Tue, 2018-04-10 at 13:22 -0400, Waiman Long wrote:
> It was observed occasionally in PowerPC systems that there was reader
> who had not been woken up but that its waiter->task had been cleared.
> 
> One probable cause of this missed wakeup may be the fact that the
> waiter->task and the task state have not been properly synchronized as
> the lock release-acquire pair of different locks in the wakeup code path
> does not provide a full memory barrier guarantee. So smp_store_mb()
> is now used to set waiter->task to NULL to provide a proper memory
> barrier for synchronization.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>

That looks right... nothing in either lock or unlock will prevent a
store going past a load.

> ---
>  kernel/locking/rwsem-xadd.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index e795908..b3c588c 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -209,6 +209,23 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
>  		smp_store_release(&waiter->task, NULL);
>  	}
>  
> +	/*
> +	 * To avoid missed wakeup of reader, we need to make sure
> +	 * that task state and waiter->task are properly synchronized.
> +	 *
> +	 *     wakeup		      sleep
> +	 *     ------		      -----
> +	 * __rwsem_mark_wake:	rwsem_down_read_failed*:
> +	 *   [S] waiter->task	  [S] set_current_state(state)
> +	 *	 MB		      MB
> +	 * try_to_wake_up:
> +	 *   [L] state		  [L] waiter->task
> +	 *
> +	 * For the wakeup path, the original lock release-acquire pair
> +	 * does not provide enough guarantee of proper synchronization.
> +	 */
> +	smp_mb();
> +
>  	adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
>  	if (list_empty(&sem->wait_list)) {
>  		/* hit end of list above */