Re: d1fc031747 ("sched/wait: assert the wait_queue_head lock is .."): EIP: __wake_up_common

[Matthew Wilcox]

Argh, forgot to cc the userfaultfd people.  Sorry.

On Thu, Dec 14, 2017 at 04:58:09AM -0800, Matthew Wilcox wrote:
> On Wed, Dec 13, 2017 at 05:03:00PM -0800, Andrew Morton wrote:
> > >     sched/wait: assert the wait_queue_head lock is held in __wake_up_common
> > >     
> > >     Better ensure we actually hold the lock using lockdep than just commenting
> > >     on it.  Due to the various exported _locked interfaces it is far too easy
> > >     to get the locking wrong.
> > 
> > I'm probably sitting on an older version.  I've dropped
> > 
> > epoll: use the waitqueue lock to protect ep->wq
> > sched/wait: assert the wait_queue_head lock is held in __wake_up_common
> 
> Looks pretty clear to me that userfaultfd is also abusing the wake_up_locked
> interfaces:
> 
>         spin_lock(&ctx->fault_pending_wqh.lock);
>         __wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, &range);
>         __wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, &range);
>         spin_unlock(&ctx->fault_pending_wqh.lock);
> 
> Sure, it's locked, but not by the lock you thought it was going to be.
> 
> There doesn't actually appear to be a bug here; fault_wqh is always serialised
> by fault_pending_wqh.lock, but lockdep can't know that.  I think this patch
> will solve the problem.
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index ac9a4e65ca49..a39bc3237b68 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -879,7 +879,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
>  	 */
>  	spin_lock(&ctx->fault_pending_wqh.lock);
>  	__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, &range);
> -	__wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, &range);
> +	__wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, &range);
>  	spin_unlock(&ctx->fault_pending_wqh.lock);
>  
>  	/* Flush pending events that may still wait on event_wqh */
> @@ -1045,7 +1045,7 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
>  			 * anyway.
>  			 */
>  			list_del(&uwq->wq.entry);
> -			__add_wait_queue(&ctx->fault_wqh, &uwq->wq);
> +			add_wait_queue(&ctx->fault_wqh, &uwq->wq);
>  
>  			write_seqcount_end(&ctx->refile_seq);
>  
> @@ -1194,7 +1194,7 @@ static void __wake_userfault(struct userfaultfd_ctx *ctx,
>  		__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL,
>  				     range);
>  	if (waitqueue_active(&ctx->fault_wqh))
> -		__wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, range);
> +		__wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, range);
>  	spin_unlock(&ctx->fault_pending_wqh.lock);
>  }
>  
> 
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