linux-kernel - Re: [PATCH] drm/sched: Fix dynamic job-flow control race

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Message-ID: <aa44441a-fca2-476d-9ad9-c4eb69a5ae83@mailbox.org>
Date: Sat, 14 Sep 2024 16:44:29 +0200
From: Michel Dänzer <michel.daenzer@...lbox.org>
To: Rob Clark <robdclark@...il.com>
Cc: dri-devel@...ts.freedesktop.org, Rob Clark <robdclark@...omium.org>,
 Asahi Lina <lina@...hilina.net>, Luben Tuikov <ltuikov89@...il.com>,
 Matthew Brost <matthew.brost@...el.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
 Danilo Krummrich <dakr@...hat.com>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/sched: Fix dynamic job-flow control race

On 2024-09-13 19:28, Rob Clark wrote:
> On Fri, Sep 13, 2024 at 10:03 AM Michel Dänzer
> <michel.daenzer@...lbox.org> wrote:
>>
>> On 2024-09-13 18:53, Rob Clark wrote:
>>> From: Rob Clark <robdclark@...omium.org>
>>>
>>> Fixes a race condition reported here: https://github.com/AsahiLinux/linux/issues/309#issuecomment-2238968609
>>>
>>> The whole premise of lockless access to a single-producer-single-
>>> consumer queue is that there is just a single producer and single
>>> consumer.  That means we can't call drm_sched_can_queue() (which is
>>> about queueing more work to the hw, not to the spsc queue) from
>>> anywhere other than the consumer (wq).
>>>
>>> This call in the producer is just an optimization to avoid scheduling
>>> the consuming worker if it cannot yet queue more work to the hw.  It
>>> is safe to drop this optimization to avoid the race condition.
>>>
>>> Suggested-by: Asahi Lina <lina@...hilina.net>
>>> Fixes: a78422e9dff3 ("drm/sched: implement dynamic job-flow control")
>>> Signed-off-by: Rob Clark <robdclark@...omium.org>
>>> ---
>>>  drivers/gpu/drm/scheduler/sched_main.c | 3 +--
>>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>> index ab53ab486fe6..1af1dbe757d5 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>> @@ -1020,8 +1020,7 @@ EXPORT_SYMBOL(drm_sched_job_cleanup);
>>>  void drm_sched_wakeup(struct drm_gpu_scheduler *sched,
>>>                     struct drm_sched_entity *entity)
>>>  {
>>> -     if (drm_sched_can_queue(sched, entity))
>>> -             drm_sched_run_job_queue(sched);
>>> +     drm_sched_run_job_queue(sched);
>>>  }
>>>
>>>  /**
>>
>> The entity parameter is unused now.
> 
> Right.. and we probably should collapse drm_sched_wakeup() and
> drm_sched_run_job_queue()..

I looked into that as well, seems fine to leave them separate for now though.


> But this fix needs to be cherry picked back to a bunch of release
> branches, so I intentionally avoided refactoring as part of the fix.

Fixing up the drm_sched_wakeup caller(s) when backporting doesn't seem like a big deal.


-- 
Earthling Michel Dänzer       \        GNOME / Xwayland / Mesa developer
https://redhat.com             \               Libre software enthusiast