Re: [PATCH 1/4] locking/ww_mutex: Fix a deadlock affecting ww_mutexes

[Daniel Vetter]

On Wed, Nov 23, 2016 at 02:00:46PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 23, 2016 at 12:25:22PM +0100, Nicolai Hähnle wrote:
> > From: Nicolai Hähnle <Nicolai.Haehnle@xxxxxxx>
> > 
> > Fix a race condition involving 4 threads and 2 ww_mutexes as indicated in
> > the following example. Acquire context stamps are ordered like the thread
> > numbers, i.e. thread #1 should back off when it encounters a mutex locked
> > by thread #0 etc.
> > 
> > Thread #0    Thread #1    Thread #2    Thread #3
> > ---------    ---------    ---------    ---------
> >                                        lock(ww)
> >                                        success
> >              lock(ww')
> >              success
> >                           lock(ww)
> >              lock(ww)        .
> >                 .            .         unlock(ww) part 1
> > lock(ww)        .            .            .
> > success         .            .            .
> >                 .            .         unlock(ww) part 2
> >                 .         back off
> > lock(ww')       .
> >    .            .
> > (stuck)      (stuck)
> > 
> > Here, unlock(ww) part 1 is the part that sets lock->base.count to 1
> > (without being protected by lock->base.wait_lock), meaning that thread #0
> > can acquire ww in the fast path or, much more likely, the medium path
> > in mutex_optimistic_spin. Since lock->base.count == 0, thread #0 then
> > won't wake up any of the waiters in ww_mutex_set_context_fastpath.
> > 
> > Then, unlock(ww) part 2 wakes up _only_the_first_ waiter of ww. This is
> > thread #2, since waiters are added at the tail. Thread #2 wakes up and
> > backs off since it sees ww owned by a context with a lower stamp.
> > 
> > Meanwhile, thread #1 is never woken up, and so it won't back off its lock
> > on ww'. So thread #0 gets stuck waiting for ww' to be released.
> > 
> > This patch fixes the deadlock by waking up all waiters in the slow path
> > of ww_mutex_unlock.
> > 
> > We have an internal test case for amdgpu which continuously submits
> > command streams from tens of threads, where all command streams reference
> > hundreds of GPU buffer objects with a lot of overlap in the buffer lists
> > between command streams. This test reliably caused a deadlock, and while I
> > haven't completely confirmed that it is exactly the scenario outlined
> > above, this patch does fix the test case.
> > 
> > v2:
> > - use wake_q_add
> > - add additional explanations
> > 
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxx>
> > Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > Cc: stable@xxxxxxxxxxxxxxx
> > Reviewed-by: Christian König <christian.koenig@xxxxxxx> (v1)
> > Signed-off-by: Nicolai Hähnle <nicolai.haehnle@xxxxxxx>
> 
> Completely and utterly fails to apply; I think this patch is based on
> code prior to the mutex rewrite.
> 
> Please rebase on tip/locking/core.
> 
> Also, is this a regression, or has this been a 'feature' of the ww_mutex
> code from early on?

Sorry forgot to mention that, but I checked. Seems to have been broken
since day 1, at least looking at the original code the wake-single-waiter
stuff is as old as the mutex code added in 2006.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch