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

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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