linux-kernel - Re: [git pull] drm for 6.1-rc1

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Date:   Fri, 7 Oct 2022 12:45:32 +1000
From:   Dave Airlie <airlied@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Arvind.Yadav@....com
Cc:     Alex Deucher <alexdeucher@...il.com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        LKML <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [git pull] drm for 6.1-rc1

On Fri, 7 Oct 2022 at 09:45, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Thu, Oct 6, 2022 at 1:25 PM Dave Airlie <airlied@...il.com> wrote:
> >
> >
> > [ 1234.778760] BUG: kernel NULL pointer dereference, address: 0000000000000088
> > [ 1234.778813] RIP: 0010:drm_sched_job_done.isra.0+0xc/0x140 [gpu_sched]
>
> As far as I can tell, that's the line
>
>         struct drm_gpu_scheduler *sched = s_fence->sched;
>
> where 's_fence' is NULL. The code is
>
>    0: 0f 1f 44 00 00        nopl   0x0(%rax,%rax,1)
>    5: 41 54                push   %r12
>    7: 55                    push   %rbp
>    8: 53                    push   %rbx
>    9: 48 89 fb              mov    %rdi,%rbx
>    c:* 48 8b af 88 00 00 00 mov    0x88(%rdi),%rbp <-- trapping instruction
>   13: f0 ff 8d f0 00 00 00 lock decl 0xf0(%rbp)
>   1a: 48 8b 85 80 01 00 00 mov    0x180(%rbp),%rax
>
> and that next 'lock decl' instruction would have been the
>
>         atomic_dec(&sched->hw_rq_count);
>
> at the top of drm_sched_job_done().
>
> Now, as to *why* you'd have a NULL s_fence, it would seem that
> drm_sched_job_cleanup() was called with an active job. Looking at that
> code, it does
>
>         if (kref_read(&job->s_fence->finished.refcount)) {
>                 /* drm_sched_job_arm() has been called */
>                 dma_fence_put(&job->s_fence->finished);
>         ...
>
> but then it does
>
>         job->s_fence = NULL;
>
> anyway, despite the job still being active. The logic of that kind of
> "fake refcount" escapes me. The above looks fundamentally racy, not to
> say pointless and wrong (a refcount is a _count_, not a flag, so there
> could be multiple references to it, what says that you can just
> decrement one of them and say "I'm done").
>
> Now, _why_ any of that happens, I have no idea. I'm just looking at
> the immediate "that pointer is NULL" thing, and reacting to what looks
> like a completely bogus refcount pattern.
>
> But that odd refcount pattern isn't new, so it's presumably some user
> on the amd gpu side that changed.
>
> The problem hasn't happened again for me, but that's not saying a lot,
> since it was very random to begin with.

I chased down the culprit to a drm sched patch, I'll send you a pull
with a revert in it.

commit e4dc45b1848bc6bcac31eb1b4ccdd7f6718b3c86
Author: Arvind Yadav <Arvind.Yadav@....com>
Date:   Wed Sep 14 22:13:20 2022 +0530

    drm/sched: Use parent fence instead of finished

    Using the parent fence instead of the finished fence
    to get the job status. This change is to avoid GPU
    scheduler timeout error which can cause GPU reset.

    Signed-off-by: Arvind Yadav <Arvind.Yadav@....com>
    Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@....com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20220914164321.2156-6-Arvind.Yadav@amd.com
    Signed-off-by: Christian König <christian.koenig@....com>

I'll let Arvind and Christian maybe work out what is going wrong there.

Dave.

>
>                  Linus