On Thu, Dec 14, 2017 at 02:10:37PM +0100, Christoph Hellwig wrote: > On Thu, Dec 14, 2017 at 04:58:09AM -0800, Matthew Wilcox wrote: > > 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. > > Or userfaultfd could just always use the waitqueue lock, similar to what > we are doing in epoll. > > But unless someone care about micro-optimizatations I'm tempted to > add your patch to the next iteration of the series.
userfaultfd is using the waitqueue lock -- it just has two waitqueues that it's protecting with the same lock. If the patch goes through as-is, try this changelog: [PATCH] userfaultfd: Use fault_wqh lock userfaultfd was using the fault_pending_wq lock to protect both fault_pending_wq and fault_wqh. With Christoph's addition of a lockdep assert to the wait queue code, that will trigger warnings (although there is no bug). Remove the warning by using __wake_up which will take the fault_wqh lock. This lock now nests inside the fault_pending_wqh lock, but that's not a problem since it was entireyl unused before. Signed-off-by: Matthew Wilcox <mawil...@microsoft.com>
