Re: [PATCH 3/3] drm/ttm: remove ttm_bo_wait_unreserved

["Thomas Hellström (VMware)" <thomas_os@shipmail.org>]

On 8/21/19 2:40 PM, Thomas Hellström (VMware) wrote:
> On 8/20/19 4:53 PM, Daniel Vetter wrote:
>> With nouveau fixed all ttm-using drives have the correct nesting of
>> mmap_sem vs dma_resv, and we can just lock the buffer.
>>
>> Assuming I didn't screw up anything with my audit of course.
>>
>> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
>> Cc: Christian Koenig <christian.koenig@amd.com>
>> Cc: Huang Rui <ray.huang@amd.com>
>> Cc: Gerd Hoffmann <kraxel@redhat.com>
>> Cc: "VMware Graphics" <linux-graphics-maintainer@vmware.com>
>> Cc: Thomas Hellstrom <thellstrom@vmware.com>
>> ---
>>   drivers/gpu/drm/ttm/ttm_bo.c    | 34 ---------------------------------
>>   drivers/gpu/drm/ttm/ttm_bo_vm.c | 26 +------------------------
>>   include/drm/ttm/ttm_bo_api.h    |  1 -
>>   3 files changed, 1 insertion(+), 60 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
>> index 20ff56f27aa4..a952dd624b06 100644
>> --- a/drivers/gpu/drm/ttm/ttm_bo.c
>> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
>> @@ -1954,37 +1954,3 @@ void ttm_bo_swapout_all(struct ttm_bo_device 
>> *bdev)
>>           ;
>>   }
>>   EXPORT_SYMBOL(ttm_bo_swapout_all);
>> -
>> -/**
>> - * ttm_bo_wait_unreserved - interruptible wait for a buffer object 
>> to become
>> - * unreserved
>> - *
>> - * @bo: Pointer to buffer
>> - */
>> -int ttm_bo_wait_unreserved(struct ttm_buffer_object *bo)
>> -{
>> -    int ret;
>> -
>> -    /*
>> -     * In the absense of a wait_unlocked API,
>> -     * Use the bo::wu_mutex to avoid triggering livelocks due to
>> -     * concurrent use of this function. Note that this use of
>> -     * bo::wu_mutex can go away if we change locking order to
>> -     * mmap_sem -> bo::reserve.
>> -     */
>> -    ret = mutex_lock_interruptible(&bo->wu_mutex);
>> -    if (unlikely(ret != 0))
>> -        return -ERESTARTSYS;
>> -    if (!dma_resv_is_locked(bo->base.resv))
>> -        goto out_unlock;
>> -    ret = dma_resv_lock_interruptible(bo->base.resv, NULL);
>> -    if (ret == -EINTR)
>> -        ret = -ERESTARTSYS;
>> -    if (unlikely(ret != 0))
>> -        goto out_unlock;
>> -    dma_resv_unlock(bo->base.resv);
>> -
>> -out_unlock:
>> -    mutex_unlock(&bo->wu_mutex);
>> -    return ret;
>> -}
>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c 
>> b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>> index 76eedb963693..505e1787aeea 100644
>> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>> @@ -125,31 +125,7 @@ static vm_fault_t ttm_bo_vm_fault(struct 
>> vm_fault *vmf)
>>           &bdev->man[bo->mem.mem_type];
>>       struct vm_area_struct cvma;
>>   -    /*
>> -     * Work around locking order reversal in fault / nopfn
>> -     * between mmap_sem and bo_reserve: Perform a trylock operation
>> -     * for reserve, and if it fails, retry the fault after waiting
>> -     * for the buffer to become unreserved.
>> -     */
>> -    if (unlikely(!dma_resv_trylock(bo->base.resv))) {
>> -        if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) {
>> -            if (!(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
>> -                ttm_bo_get(bo);
>> - up_read(&vmf->vma->vm_mm->mmap_sem);
>> -                (void) ttm_bo_wait_unreserved(bo);
>> -                ttm_bo_put(bo);
>> -            }
>> -
>> -            return VM_FAULT_RETRY;
>> -        }
>> -
>> -        /*
>> -         * If we'd want to change locking order to
>> -         * mmap_sem -> bo::reserve, we'd use a blocking reserve here
>> -         * instead of retrying the fault...
>> -         */
>
> I think you should justify why the above code is removed, since the 
> comments actually outlines what to do if we change locking order.
>
> The code that's removed above is not for adjusting locking orders but 
> to decrease the mm latency by releasing the mmap_sem while waiting for 
> bo reserve which in turn might be waiting for GPU. At a minimum we 
> should have a separate patch with justification.
>
> Note that the caller here ensures locking progress by adjusting the 
> RETRY flags after a retry.

And this last sentence also means we still can remove the wu-mutex when 
the locking order is changed, since the chance of livelock is goes away. 
IIRC only a single RETRY spin is allowed by the mm core.

/Thomas

>
> /Thomas
>
>
>> -        return VM_FAULT_NOPAGE;
>> -    }
>> +    dma_resv_lock(bo->base.resv, NULL);
>>         /*
>>        * Refuse to fault imported pages. This should be handled
>> diff --git a/include/drm/ttm/ttm_bo_api.h b/include/drm/ttm/ttm_bo_api.h
>> index 43c4929a2171..6b50e624e3e2 100644
>> --- a/include/drm/ttm/ttm_bo_api.h
>> +++ b/include/drm/ttm/ttm_bo_api.h
>> @@ -765,7 +765,6 @@ ssize_t ttm_bo_io(struct ttm_bo_device *bdev, 
>> struct file *filp,
>>   int ttm_bo_swapout(struct ttm_bo_global *glob,
>>               struct ttm_operation_ctx *ctx);
>>   void ttm_bo_swapout_all(struct ttm_bo_device *bdev);
>> -int ttm_bo_wait_unreserved(struct ttm_buffer_object *bo);
>>     /**
>>    * ttm_bo_uses_embedded_gem_object - check if the given bo uses the
>
>

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