[AMD Official Use Only - Internal Distribution Only]
Am 12.08.20 um 03:19 schrieb Li, Dennis:
> [AMD Official Use Only - Internal Distribution Only]
>
> Hi, Felix,
>
> Re: 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.
> [Dennis Li] Thanks that you find the potential issue, I will change it in version 2.
>
> Re: 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?
> [Dennis Li] Yes, amdgpu_gart_bind will flush HDP and TLB. I have considered to only protect amdgpu_ttm_alloc_gart before.
That access is irrelevant and the lock should be removed or changed into a trylock.
See we need the HDP flush only because the hardware could have accessed the data before.
But after a GPU reset the HDP is known to be clean, so this doesn't need any protection.
> But I worry other functions will access hardware in the future. Therefore I select an aggressive approach which lock reset_sem at the beginning of entry functions of amdgpu driver.
This is not a good idea. We used to have such a global lock before and removed it because it caused all kind of problems.
[Dennis Li] okay. If you don't agree this aggressive approach. I will change to protect amdgpu_ttm_alloc_gart only.
When was this added? Looks like it slipped under my radar or I wasn't awake enough at that moment.
Christian.
>
> Best Regards
> Dennis Li
> -----Original Message-----
> From: Kuehling, Felix <Felix.Kuehling@xxxxxxx>
> Sent: Tuesday, August 11, 2020 9:57 PM
> To: Li, Dennis <Dennis.Li@xxxxxxx>; amd-gfx@xxxxxxxxxxxxxxxxxxxxx;
> Deucher, Alexander <Alexander.Deucher@xxxxxxx>; Zhang, Hawking
> <Hawking.Zhang@xxxxxxx>; Koenig, Christian <Christian.Koenig@xxxxxxx>
> Subject: Re: [PATCH] drm/amdgpu: fix a potential circular locking
> dependency
>
> 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?
>
> Regards,
> Felix
>
>
>> Signed-off-by: Dennis Li <Dennis.Li@xxxxxxx>
>>
>> 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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