RE: [PATCH] drm/amdgpu: fix a potential circular locking dependency

["Li, Dennis" <Dennis.Li@amd.com>]

[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. 

Best Regards
Dennis Li
-----Original Message-----
From: Koenig, Christian <Christian.Koenig@amd.com> 
Sent: Wednesday, August 12, 2020 12:15 AM
To: Kuehling, Felix <Felix.Kuehling@amd.com>; Li, Dennis <Dennis.Li@amd.com>; amd-gfx@lists.freedesktop.org; Deucher, Alexander <Alexander.Deucher@amd.com>; Zhang, Hawking <Hawking.Zhang@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@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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