Re: [PATCH] drm/amdgpu: hold the reference of finished fence

["Christian König" <christian.koenig@amd.com>]

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There is a misunderstanding here:

> Did you find out why the zero refcount on the finished fence happens
> before the fence was signaled ?

The refcount on the finished fence doesn't become zero before it is 
signaled, it becomes zero while it is signaled.

CPU 1 calls dma_fence_signal(fence) without holding a reference to the 
fence. CPU 2 at the same time checks if the fence is signaled and frees 
the last reference because it find the signaled flag to be set.

The problem is now that dma_fence_signal() wants to set the timestamp 
after setting the signaled flag and now races with freeing the memory.

That's a really hard to hit problem, but it indeed seems to be possible.

Christian.

Am 24.03.20 um 15:52 schrieb Andrey Grodzovsky:
>
> This is only for the guilty job which was removed from the 
> ring_mirror_list due to completion and hence will not be resubmitted 
> by recovery and will not be freed by the usual flow in 
> drm_sched_get_cleanup_job (see drm_sched_stop)
>
> Andrey
>
> On 3/24/20 10:45 AM, Pan, Xinhui wrote:
>>
>> [AMD Official Use Only - Internal Distribution Only]
>>
>>
>> Does this issue occur when gpu recovery?
>> I just check the code,  fence timedout will free job and put its 
>> fence. but gpu recovery might resubmit job.
>> Correct me if I am wrong.
>> ------------------------------------------------------------------------
>> *From:* amd-gfx <amd-gfx-bounces@lists.freedesktop.org> on behalf of 
>> Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
>> *Sent:* Tuesday, March 24, 2020 11:40:06 AM
>> *To:* Tao, Yintian <Yintian.Tao@amd.com>; Koenig, Christian 
>> <Christian.Koenig@amd.com>; Deucher, Alexander 
>> <Alexander.Deucher@amd.com>
>> *Cc:* amd-gfx@lists.freedesktop.org <amd-gfx@lists.freedesktop.org>
>> *Subject:* Re: [PATCH] drm/amdgpu: hold the reference of finished fence
>>
>> On 3/23/20 10:22 AM, Yintian Tao wrote:
>> > There is one one corner case at dma_fence_signal_locked
>> > which will raise the NULL pointer problem just like below.
>> > ->dma_fence_signal
>> >      ->dma_fence_signal_locked
>> >        ->test_and_set_bit
>> > here trigger dma_fence_release happen due to the zero of fence 
>> refcount.
>>
>>
>> Did you find out why the zero refcount on the finished fence happens
>> before the fence was signaled ? The finished fence is created with
>> refcount set to 1 in drm_sched_fence_create->dma_fence_init and then the
>> refcount is decremented in
>> drm_sched_main->amdgpu_job_free_cb->drm_sched_job_cleanup. This should
>> only happen after fence is already signaled (see
>> drm_sched_get_cleanup_job). On top of that the finished fence is
>> referenced from other places (e.g. entity->last_scheduled e.t.c)...
>>
>>
>> >
>> > ->dma_fence_put
>> >      ->dma_fence_release
>> >        ->drm_sched_fence_release_scheduled
>> >            ->call_rcu
>> > here make the union fled “cb_list” at finished fence
>> > to NULL because struct rcu_head contains two pointer
>> > which is same as struct list_head cb_list
>> >
>> > Therefore, to hold the reference of finished fence at 
>> drm_sched_process_job
>> > to prevent the null pointer during finished fence dma_fence_signal
>> >
>> > [  732.912867] BUG: kernel NULL pointer dereference, address: 
>> 0000000000000008
>> > [  732.914815] #PF: supervisor write access in kernel mode
>> > [  732.915731] #PF: error_code(0x0002) - not-present page
>> > [  732.916621] PGD 0 P4D 0
>> > [  732.917072] Oops: 0002 [#1] SMP PTI
>> > [  732.917682] CPU: 7 PID: 0 Comm: swapper/7 Tainted: G           
>> OE     5.4.0-rc7 #1
>> > [  732.918980] Hardware name: QEMU Standard PC (i440FX + PIIX, 
>> 1996), BIOS rel-1.8.2-0-g33fbe13 by qemu-project.org 04/01/2014
>> > [  732.920906] RIP: 0010:dma_fence_signal_locked+0x3e/0x100
>> > [  732.938569] Call Trace:
>> > [  732.939003]  <IRQ>
>> > [  732.939364]  dma_fence_signal+0x29/0x50
>> > [  732.940036] drm_sched_fence_finished+0x12/0x20 [gpu_sched]
>> > [  732.940996]  drm_sched_process_job+0x34/0xa0 [gpu_sched]
>> > [  732.941910] dma_fence_signal_locked+0x85/0x100
>> > [  732.942692]  dma_fence_signal+0x29/0x50
>> > [  732.943457]  amdgpu_fence_process+0x99/0x120 [amdgpu]
>> > [  732.944393] sdma_v4_0_process_trap_irq+0x81/0xa0 [amdgpu]
>> >
>> > v2: hold the finished fence at drm_sched_process_job instead of
>> >      amdgpu_fence_process
>> > v3: resume the blank line
>> >
>> > Signed-off-by: Yintian Tao <yttao@amd.com>
>> > ---
>> >   drivers/gpu/drm/scheduler/sched_main.c | 2 ++
>> >   1 file changed, 2 insertions(+)
>> >
>> > diff --git a/drivers/gpu/drm/scheduler/sched_main.c 
>> b/drivers/gpu/drm/scheduler/sched_main.c
>> > index a18eabf692e4..8e731ed0d9d9 100644
>> > --- a/drivers/gpu/drm/scheduler/sched_main.c
>> > +++ b/drivers/gpu/drm/scheduler/sched_main.c
>> > @@ -651,7 +651,9 @@ static void drm_sched_process_job(struct 
>> dma_fence *f, struct dma_fence_cb *cb)
>> >
>> >        trace_drm_sched_process_job(s_fence);
>> >
>> > +     dma_fence_get(&s_fence->finished);
>> >        drm_sched_fence_finished(s_fence);
>>
>>
>> If the fence was already released during call to
>> drm_sched_fence_finished->dma_fence_signal->... why is it safe to
>> reference the s_fence just before that call ? Can't it already be
>> released by this time ?
>>
>> Andrey
>>
>>
>>
>> > +     dma_fence_put(&s_fence->finished);
>> > wake_up_interruptible(&sched->wake_up_worker);
>> >   }
>> >
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