force app kill patch

[Liu, Monk]

*Correctio for the scenario *

After we move fence_put(entity->last_sched) out of the fini_status check:


A potential race issue for the scenario:


1.        Drm_sched_entity_fini(): it exit right after entity->job_queue empty, [ but that time scheduler is not fast enough to deal with this entity now ]

2.        Drm_sched_entity_cleanup() : it call dma_fence_put(entity->last_scheduled)  [ but this time entity->last_scheduled  actually points to the fence prior to the real last one ]

3.        Scheduler_main() now dealing with this entity: it call dma_fence_put(entity->last_scheduled)    [   Now this fence get double put !!!  ]

4.        Scheduler_main() now call dma_fence_get() on the *real* last one !

So eventually the real last one fence triggers memory leak and more critical the double put fence cause NULL pointer access

/Monk

From: Liu, Monk
Sent: 2018年4月18日 15:11
To: Koenig, Christian <Christian.Koenig at amd.com>; Deng, Emily <Emily.Deng at amd.com>
Cc: amd-gfx at lists.freedesktop.org
Subject: force app kill patch

Hi Christian & Emily

I think the v4 fix for “fix force app kill hang” is still not good enough:


First:
See that in “sched_entity_fini”, we only call dma_fence_put(entity->last_scheduled” under the condition of “If (entity->fini_status)”, so
This way there is memory leak for the case of “entity->fini_stats ==0”


Second:
If we move dma_fence_put(entity->last_scheduled) out of the condition of “if (entity->fini_status)”, the memory leak issue can be fixed
But there will be kernel NULL pointer access, I think the time you call dma_fence_put(entity->last_scheduled”) may actually executed *not*
On the last scheduled fence of this entity, because it is run without “thread_park/unpark” pair which to make sure scheduler not dealing this entity

So with certain race issue, here is the scenario:


1.        scheduler is doing the dma_fence_put() on the 1st fence,

2.        scheduler set entity->last_scheduled to 1st fence

3.        now sched_entity_fini() run, and it call dma_fence_put() on entity->last_scheduled

4.        now this 1st fence is actually put double time and the real last fence won’t get put by expected


any idea?


/Monk
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