From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx3-rdu2.redhat.com ([66.187.233.73]:49704 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726505AbeIJNsu (ORCPT ); Mon, 10 Sep 2018 09:48:50 -0400 Subject: Re: [RFC] pipe: prevent compiler reordering in pipe_poll To: Eric Wong , Al Viro Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org References: <20180824225431.tpaxuck7idgnj3b7@dcvr> From: Paolo Bonzini Message-ID: <817dd145-fba5-d5e8-26b8-746b9bab4dd9@redhat.com> Date: Mon, 10 Sep 2018 10:55:47 +0200 MIME-Version: 1.0 In-Reply-To: <20180824225431.tpaxuck7idgnj3b7@dcvr> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On 25/08/2018 00:54, Eric Wong wrote: > The pipe_poll function does not use locks, and adding an entry > to the waitqueue is not guaranteed to happen before pipe->nrbufs > (or other fields) are read, leading to missed wakeups. > > Looking at Ruby CI build logs and backtraces, I've noticed > occasional instances where processes are stuck in select(2) or > ppoll(2) with a pipe. > > I don't have access to the systems where this is happening to > test/reproduce the problem, and haven't been able to reproduce > it locally on less-powerful hardware, either. However, it seems > like a problem based on similar comments in > fs/eventfd.c::eventfd_poll made by Paolo. The documentation change can be useful, but if you add a compiler barrier you should also mention why reordering at the processor level is okay. In this case, processor-level reordering is okay because (just like in fs/eventfd.c) poll_wait acts as an acquire barrier. *However* I would be surprised if the scenario (even the one in fs/eventfd.c) can actually happen, and I don't think the compiler barrier is useful; there's no reason why the compiler should think that it can hoist the reads above poll_wait. In fact, there is a big difference between READ_ONCE() and barrier() for whoever reads the code, which makes the code after your patch worse than before. READ_ONCE() means "I know I am accessing this variable outside a lock". barrier() means one of two things: 1) "I know what I am doing can trick the compiler, and I don't want that to happen"; 2) "I am synchronizing against other things happening on this CPU" such as interrupts. In this case you are not doing any of the two. Paolo > Signed-off-by: Eric Wong > Cc: Paolo Bonzini > --- > fs/pipe.c | 32 ++++++++++++++++++++++++++++++-- > 1 file changed, 30 insertions(+), 2 deletions(-) > > diff --git a/fs/pipe.c b/fs/pipe.c > index 39d6f431da83..1a904d941cf1 100644 > --- a/fs/pipe.c > +++ b/fs/pipe.c > @@ -509,7 +509,7 @@ static long pipe_ioctl(struct file *filp, unsigned int cmd, unsigned long arg) > } > } > > -/* No kernel lock held - fine */ > +/* No kernel lock held - fine, but a compiler barrier is required */ > static __poll_t > pipe_poll(struct file *filp, poll_table *wait) > { > @@ -519,7 +519,35 @@ pipe_poll(struct file *filp, poll_table *wait) > > poll_wait(filp, &pipe->wait, wait); > > - /* Reading only -- no need for acquiring the semaphore. */ > + /* > + * Reading only -- no need for acquiring the semaphore, but > + * we need a compiler barrier to ensure the compiler does > + * not reorder reads to pipe->nrbufs, pipe->writers, > + * pipe->readers, filp->f_version, pipe->w_counter, and > + * pipe->buffers before poll_wait to avoid missing wakeups > + * from compiler reordering. In other words, we need to > + * prevent the following situation: > + * > + * pipe_poll pipe_write > + * ----------------- ------------ > + * nrbufs = pipe->nrbufs (INVALID!) > + * > + * __pipe_lock > + * pipe->nrbufs = ++bufs; > + * __pipe_unlock > + * wake_up_interruptible_sync_poll > + * pipe->wait is empty, no wakeup > + * > + * lock pipe->wait.lock (in poll_wait) > + * __add_wait_queue > + * unlock pipe->wait.lock > + * > + * // pipe->nrbufs should be read here, NOT above > + * > + * pipe_poll returns 0 (WRONG) > + */ > + barrier(); > + > nrbufs = pipe->nrbufs; > mask = 0; > if (filp->f_mode & FMODE_READ) { >