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

[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?

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;
>  }