Messages in this thread Patch in this message | | | Date | Thu, 14 Dec 2017 04:58:09 -0800 | From | Matthew Wilcox <> | Subject | Re: d1fc031747 ("sched/wait: assert the wait_queue_head lock is .."): EIP: __wake_up_common |
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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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