Re: [PATCH] drm/i915: Fix unhandled deadlock in grab_vma()

[Thomas Hellström]

Hi, Mani.

On 11/14/22 03:16, Mani Milani wrote:
> Thank you for your comments.
>
> To Thomas's point, the crash always seems to happen when the following
> sequence of events occurs:
>
> 1. When inside "i915_gem_evict_vm()", the call to
> "i915_gem_object_trylock(vma->obj, ww)" fails (due to deadlock), and
> eviction of a vma is skipped as a result. Basically if the code
> reaches here:
> https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/i915/i915_gem_evict.c#L468
> And here is the stack dump for this scenario:
>      Call Trace:
>      <TASK>
>      dump_stack_lvl+0x68/0x95
>      i915_gem_evict_vm+0x1d2/0x369
>      eb_validate_vmas+0x54a/0x6ae
>      eb_relocate_parse+0x4b/0xdb
>      i915_gem_execbuffer2_ioctl+0x6f5/0xab6
>      ? i915_gem_object_prepare_write+0xfb/0xfb
>      drm_ioctl_kernel+0xda/0x14d
>      drm_ioctl+0x27f/0x3b7
>      ? i915_gem_object_prepare_write+0xfb/0xfb
>      __se_sys_ioctl+0x7a/0xbc
>      do_syscall_64+0x56/0xa1
>      ? exit_to_user_mode_prepare+0x3d/0x8c
>      entry_SYSCALL_64_after_hwframe+0x61/0xcb
>      RIP: 0033:0x78302de5fae7
>      Code: c0 0f 89 74 ff ff ff 48 83 c4 08 49 c7 c4 ff ff ff ff 5b 4c
> 89 e0 41 5c 41 5d 5d c3 0f 1f 80 00 00 00 00 b8 10 00 00 00 0f 05 <48>
> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 51 c3 0c 00 f7 d8 64 89 01 48
>      RSP: 002b:00007ffe64b87f78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
>      RAX: ffffffffffffffda RBX: 000003cc00470000 RCX: 000078302de5fae7
>      RDX: 00007ffe64b87fd0 RSI: 0000000040406469 RDI: 000000000000000d
>      RBP: 00007ffe64b87fa0 R08: 0000000000000013 R09: 000003cc004d0950
>      R10: 0000000000000200 R11: 0000000000000246 R12: 000000000000000d
>      R13: 0000000000000000 R14: 00007ffe64b87fd0 R15: 0000000040406469
>      </TASK>
> It is worth noting that "i915_gem_evict_vm()" still returns success in
> this case.
>
> 2. After step 1 occurs, the next call to "grab_vma()" always fails
> (with "i915_gem_object_trylock(vma->obj, ww)" failing also due to
> deadlock), which then results in the crash.
> Here is the stack dump for this scenario:
>      Call Trace:
>      <TASK>
>      dump_stack_lvl+0x68/0x95
>      grab_vma+0x6c/0xd0
>      i915_gem_evict_for_node+0x178/0x23b
>      i915_gem_gtt_reserve+0x5a/0x82
>      i915_vma_insert+0x295/0x29e
>      i915_vma_pin_ww+0x41e/0x5c7
>      eb_validate_vmas+0x5f5/0x6ae
>      eb_relocate_parse+0x4b/0xdb
>      i915_gem_execbuffer2_ioctl+0x6f5/0xab6
>      ? i915_gem_object_prepare_write+0xfb/0xfb
>      drm_ioctl_kernel+0xda/0x14d
>      drm_ioctl+0x27f/0x3b7
>      ? i915_gem_object_prepare_write+0xfb/0xfb
>      __se_sys_ioctl+0x7a/0xbc
>      do_syscall_64+0x56/0xa1
>      ? exit_to_user_mode_prepare+0x3d/0x8c
>      entry_SYSCALL_64_after_hwframe+0x61/0xcb
>      RIP: 0033:0x78302de5fae7
>      Code: c0 0f 89 74 ff ff ff 48 83 c4 08 49 c7 c4 ff ff ff ff 5b 4c
> 89 e0 41 5c 41 5d 5d c3 0f 1f 80 00 00 00 00 b8 10 00 00 00 0f 05 <48>
> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 51 c3 0c 00 f7 d8 64 89 01 48
>      RSP: 002b:00007ffe64b87f78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
>      RAX: ffffffffffffffda RBX: 000003cc00470000 RCX: 000078302de5fae7
>      RDX: 00007ffe64b87fd0 RSI: 0000000040406469 RDI: 000000000000000d
>      RBP: 00007ffe64b87fa0 R08: 0000000000000013 R09: 000003cc004d0950
>      R10: 0000000000000200 R11: 0000000000000246 R12: 000000000000000d
>      R13: 0000000000000000 R14: 00007ffe64b87fd0 R15: 0000000040406469
>      </TASK>
>
> My Notes:
> - I verified the two "i915_gem_object_trylock()" failures I mentioned
> above are due to deadlock by slightly modifying the code to call
> "i915_gem_object_lock()" only in those exact cases and subsequent to
> the trylock failure, only to look at the return error code.
> - The two cases mentioned above, are the only cases where
> "i915_gem_object_trylock(obj, ww)" is called with the second argument
> not being forced to NULL.
> - When in either of the two cases above (i.e. inside "grab_vma()" or
> "i915_gem_evict_vm") I replace calling "i915_gem_object_trylock" with
> "i915_gem_object_lock", the issue gets resolved (because deadlock is
> detected and resolved).
>
> So if this could matches the design better, another solution could be
> for "grab_vma" to continue to call "i915_gem_object_trylock", but for
> "i915_gem_evict_vm" to call "i915_gem_object_lock" instead.

No, i915_gem_object_lock() is not allowed when the vm mutex is held.


> Further info:
> - Would you like any further info on the crash? If so, could you
> please advise 1) what exactly you need and 2) how I can share with you
> especially if it is big dumps?

Yes, I would like to know how the crash manifests itself. Is it a kernel 
BUG or a kernel WARNING or is it the user-space application that crashes 
due to receiveing an -ENOSPC?

Thanks,

Thomas



> Thanks.