[PATCH] drm/amdgpu: fix a potential circular locking dependency
Christian König
christian.koenig at amd.com
Wed Aug 12 10:04:55 UTC 2020
Am 12.08.20 um 12:02 schrieb Li, Dennis:
> [AMD Official Use Only - Internal Distribution Only]
>
> Am 12.08.20 um 11:23 schrieb Li, Dennis:
>> [AMD Official Use Only - Internal Distribution Only]
>>
>> Am 12.08.20 um 03:33 schrieb Li, Dennis:
>>> [AMD Official Use Only - Internal Distribution Only]
>>>
>>> Hi, Christian,
>>>
>>> Re: I was wondering the same thing for the amdgpu_gem_va_ioctl() as well. We shouldn't have any hardware access here, so taking the reset_sem looks like overkill to me.
>>>
>>> [Dennis Li] amdgpu_vm_bo_unmap, amdgpu_vm_bo_clear_mappings, amdgpu_vm_bo_replace_map and amdgpu_gem_va_update_vm all a chance to access hardware.
>> This is complete nonsense. The functions intentionally work through the scheduler to avoid accessing the hardware directly for exactly that reason.
>>
>> The only hardware access we have here is the HDP flush and that can fail in the case of a GPU reset without causing problems.
>>
>> [Dennis Li] amdgpu_vm_bo_clear_mappings -> amdgpu_vm_prt_get ->
>> amdgpu_vm_update_prt_state -> gmc_v8_0_set_prt
> That is for pre gfx9 hardware and only called once during initial enabling of the feature.
>
> Please remove that locking again since it is clearly completely against the driver design.
>
> [Dennis Li] okay, if you agree, I will change to only protect amdgpu_gem_va_update_vm in this function.
Better even only protect the amdgpu_vm_update_prt_state() function.
Christian.
>
> Christian.
>
>> Regards,
>> Christian.
>>
>>> Best Regards
>>> Dennis Li
>>> -----Original Message-----
>>> From: Koenig, Christian <Christian.Koenig at amd.com>
>>> Sent: Wednesday, August 12, 2020 12:15 AM
>>> To: Kuehling, Felix <Felix.Kuehling at amd.com>; Li, Dennis
>>> <Dennis.Li at amd.com>; amd-gfx at lists.freedesktop.org; Deucher,
>>> Alexander <Alexander.Deucher at amd.com>; Zhang, Hawking
>>> <Hawking.Zhang at amd.com>
>>> Subject: Re: [PATCH] drm/amdgpu: fix a potential circular locking
>>> dependency
>>>
>>> Am 11.08.20 um 15:57 schrieb Felix Kuehling:
>>>> Am 2020-08-11 um 5:32 a.m. schrieb Dennis Li:
>>>>> [ 653.902305]
>>>>> ======================================================
>>>>> [ 653.902928] WARNING: possible circular locking dependency detected
>>>>> [ 653.903517] 5.6.0-deli-v5.6-2848-g3f3109b0e75f #1 Tainted: G OE
>>>>> [ 653.904098]
>>>>> ------------------------------------------------------
>>>>> [ 653.904675] amdgpu_test/3975 is trying to acquire lock:
>>>>> [ 653.905241] ffff97848f8647a0 (&adev->reset_sem){.+.+}, at:
>>>>> amdgpu_gem_va_ioctl+0x286/0x4f0 [amdgpu] [ 653.905953]
>>>>> but task is already holding lock:
>>>>> [ 653.907087] ffff9744adbee1f8 (reservation_ww_class_mutex){+.+.},
>>>>> at: ttm_eu_reserve_buffers+0x1ae/0x520 [ttm] [ 653.907694]
>>>>> which lock already depends on the new lock.
>>>>>
>>>>> [ 653.909423]
>>>>> the existing dependency chain (in reverse order) is:
>>>>> [ 653.910594]
>>>>> -> #1 (reservation_ww_class_mutex){+.+.}:
>>>>> [ 653.911759] __ww_mutex_lock.constprop.15+0xca/0x1120
>>>>> [ 653.912350] ww_mutex_lock+0x73/0x80
>>>>> [ 653.913044] amdgpu_amdkfd_alloc_gtt_mem+0xde/0x380 [amdgpu]
>>>>> [ 653.913724] kgd2kfd_device_init+0x13f/0x5e0 [amdgpu]
>>>>> [ 653.914388] amdgpu_amdkfd_device_init+0x155/0x190 [amdgpu]
>>>>> [ 653.915033] amdgpu_device_init+0x1303/0x1e10 [amdgpu]
>>>>> [ 653.915685] amdgpu_driver_load_kms+0x5c/0x2c0 [amdgpu]
>>>>> [ 653.916349] amdgpu_pci_probe+0x11d/0x200 [amdgpu]
>>>>> [ 653.916959] local_pci_probe+0x47/0xa0
>>>>> [ 653.917570] work_for_cpu_fn+0x1a/0x30
>>>>> [ 653.918184] process_one_work+0x29e/0x630
>>>>> [ 653.918803] worker_thread+0x22b/0x3f0
>>>>> [ 653.919427] kthread+0x12f/0x150
>>>>> [ 653.920047] ret_from_fork+0x3a/0x50
>>>>> [ 653.920661]
>>>>> -> #0 (&adev->reset_sem){.+.+}:
>>>>> [ 653.921893] __lock_acquire+0x13ec/0x16e0
>>>>> [ 653.922531] lock_acquire+0xb8/0x1c0
>>>>> [ 653.923174] down_read+0x48/0x230
>>>>> [ 653.923886] amdgpu_gem_va_ioctl+0x286/0x4f0 [amdgpu]
>>>>> [ 653.924588] drm_ioctl_kernel+0xb6/0x100 [drm]
>>>>> [ 653.925283] drm_ioctl+0x389/0x450 [drm]
>>>>> [ 653.926013] amdgpu_drm_ioctl+0x4f/0x80 [amdgpu]
>>>>> [ 653.926686] ksys_ioctl+0x98/0xb0
>>>>> [ 653.927357] __x64_sys_ioctl+0x1a/0x20
>>>>> [ 653.928030] do_syscall_64+0x5f/0x250
>>>>> [ 653.928697] entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>>>> [ 653.929373]
>>>>> other info that might help us debug this:
>>>>>
>>>>> [ 653.931356] Possible unsafe locking scenario:
>>>>>
>>>>> [ 653.932647] CPU0 CPU1
>>>>> [ 653.933287] ---- ----
>>>>> [ 653.933911] lock(reservation_ww_class_mutex);
>>>>> [ 653.934530] lock(&adev->reset_sem);
>>>>> [ 653.935154] lock(reservation_ww_class_mutex);
>>>>> [ 653.935766] lock(&adev->reset_sem);
>>>>> [ 653.936360]
>>>>> *** DEADLOCK ***
>>>>>
>>>>> [ 653.938028] 2 locks held by amdgpu_test/3975:
>>>>> [ 653.938574] #0: ffffb2a862d6bcd0
>>>>> (reservation_ww_class_acquire){+.+.}, at:
>>>>> amdgpu_gem_va_ioctl+0x39b/0x4f0 [amdgpu] [ 653.939233] #1:
>>>>> ffff9744adbee1f8 (reservation_ww_class_mutex){+.+.}, at:
>>>>> ttm_eu_reserve_buffers+0x1ae/0x520 [ttm]
>>>>>
>>>>> change the order of reservation_ww_class_mutex and adev->reset_sem
>>>>> in amdgpu_gem_va_ioctl the same as ones in
>>>>> amdgpu_amdkfd_alloc_gtt_mem, to avoid potential dead lock.
>>>> It may be better to fix it the other way around in
>>>> amdgpu_amdkfd_alloc_gtt_mem. Always take the reset_sem inside the
>>>> reservation. Otherwise you will never be able to take the reset_sem
>>>> while any BOs are reserved. That's probably going to cause you other
>>>> problems later.
>>>>
>>>> That makes me wonder, why do you need the reset_sem in
>>>> amdgpu_amdkfd_alloc_gtt_mem in the first place? There is no obvious
>>>> hardware access in that function. Is it for amdgpu_ttm_alloc_gart
>>>> updating the GART table through HDP?
>>> I was wondering the same thing for the amdgpu_gem_va_ioctl() as well.
>>>
>>> We shouldn't have any hardware access here, so taking the reset_sem looks like overkill to me.
>>>
>>> Christian.
>>>
>>>> Regards,
>>>> Felix
>>>>
>>>>
>>>>> Signed-off-by: Dennis Li <Dennis.Li at amd.com>
>>>>>
>>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
>>>>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
>>>>> index ee1e8fff83b2..fc889c477696 100644
>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
>>>>> @@ -652,6 +652,8 @@ int amdgpu_gem_va_ioctl(struct drm_device *dev, void *data,
>>>>> abo = NULL;
>>>>> }
>>>>>
>>>>> + down_read(&adev->reset_sem);
>>>>> +
>>>>> amdgpu_vm_get_pd_bo(&fpriv->vm, &list, &vm_pd);
>>>>>
>>>>> r = ttm_eu_reserve_buffers(&ticket, &list, true, &duplicates);
>>>>> @@
>>>>> -670,8 +672,6 @@ int amdgpu_gem_va_ioctl(struct drm_device *dev, void *data,
>>>>> bo_va = NULL;
>>>>> }
>>>>>
>>>>> - down_read(&adev->reset_sem);
>>>>> -
>>>>> switch (args->operation) {
>>>>> case AMDGPU_VA_OP_MAP:
>>>>> va_flags = amdgpu_gem_va_map_flags(adev, args->flags); @@
>>>>> -701,12
>>>>> +701,11 @@ int amdgpu_gem_va_ioctl(struct drm_device *dev, void
>>>>> +*data,
>>>>> amdgpu_gem_va_update_vm(adev, &fpriv->vm, bo_va,
>>>>> args->operation);
>>>>>
>>>>> - up_read(&adev->reset_sem);
>>>>> -
>>>>> error_backoff:
>>>>> ttm_eu_backoff_reservation(&ticket, &list);
>>>>>
>>>>> error_unref:
>>>>> + up_read(&adev->reset_sem);
>>>>> drm_gem_object_put_unlocked(gobj);
>>>>> return r;
>>>>> }
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