Re: [PATCH] drm/ttm: remove ttm_bo_wait_unreserved

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Aug 22, 2019 at 07:56:56AM +0000, Koenig, Christian wrote:
> Am 22.08.19 um 08:49 schrieb Daniel Vetter:
> > 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.
> >
> > v2:
> > - Dont forget wu_mutex (Christian König)
> > - Keep the mmap_sem-less wait optimization (Thomas)
> > - Use _lock_interruptible to be good citizens (Thomas)
> >
> > Reviewed-by: Christian König <christian.koenig@amd.com>

btw I realized I didn't remove your r-b, since v1 was broken.

For formality, can you pls reaffirm, or still something broken?

Also from the other thread: Reviewed-by: Thomas Hellström <thellstrom@vmware.com>

Thanks, Daniel

> > 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      | 36 -------------------------------
> >   drivers/gpu/drm/ttm/ttm_bo_util.c |  1 -
> >   drivers/gpu/drm/ttm/ttm_bo_vm.c   | 18 +++++-----------
> >   include/drm/ttm/ttm_bo_api.h      |  4 ----
> >   4 files changed, 5 insertions(+), 54 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
> > index 20ff56f27aa4..d1ce5d315d5b 100644
> > --- a/drivers/gpu/drm/ttm/ttm_bo.c
> > +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> > @@ -162,7 +162,6 @@ static void ttm_bo_release_list(struct kref *list_kref)
> >   	dma_fence_put(bo->moving);
> >   	if (!ttm_bo_uses_embedded_gem_object(bo))
> >   		dma_resv_fini(&bo->base._resv);
> > -	mutex_destroy(&bo->wu_mutex);
> >   	bo->destroy(bo);
> >   	ttm_mem_global_free(bdev->glob->mem_glob, acc_size);
> >   }
> > @@ -1319,7 +1318,6 @@ int ttm_bo_init_reserved(struct ttm_bo_device *bdev,
> >   	INIT_LIST_HEAD(&bo->ddestroy);
> >   	INIT_LIST_HEAD(&bo->swap);
> >   	INIT_LIST_HEAD(&bo->io_reserve_lru);
> > -	mutex_init(&bo->wu_mutex);
> >   	bo->bdev = bdev;
> >   	bo->type = type;
> >   	bo->num_pages = num_pages;
> > @@ -1954,37 +1952,3 @@ 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_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
> > index fe81c565e7ef..82ea26a49959 100644
> > --- a/drivers/gpu/drm/ttm/ttm_bo_util.c
> > +++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
> > @@ -508,7 +508,6 @@ static int ttm_buffer_object_transfer(struct ttm_buffer_object *bo,
> >   	INIT_LIST_HEAD(&fbo->base.lru);
> >   	INIT_LIST_HEAD(&fbo->base.swap);
> >   	INIT_LIST_HEAD(&fbo->base.io_reserve_lru);
> > -	mutex_init(&fbo->base.wu_mutex);
> >   	fbo->base.moving = NULL;
> >   	drm_vma_node_reset(&fbo->base.base.vma_node);
> >   	atomic_set(&fbo->base.cpu_writers, 0);
> > diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > index 76eedb963693..a61a35e57d1c 100644
> > --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > @@ -125,30 +125,22 @@ 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)) {
> 
> Not an expert on fault handling, but shouldn't this now be one if?
> 
> E.g. if FAULT_FLAG_RETRY_NOWAIT is set we should return VM_FAULT_NOPAGE 
> instead of VM_FAULT_RETRY.
> 
> But really take that with a grain of salt,
> Christian.
> 
> >   				ttm_bo_get(bo);
> >   				up_read(&vmf->vma->vm_mm->mmap_sem);
> > -				(void) ttm_bo_wait_unreserved(bo);
> > +				if (!dma_resv_lock_interruptible(bo->base.resv,
> > +								 NULL))
> > +					dma_resv_unlock(bo->base.resv);
> >   				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...
> > -		 */
> > -		return VM_FAULT_NOPAGE;
> > +		if (dma_resv_lock_interruptible(bo->base.resv, NULL))
> > +			return VM_FAULT_NOPAGE;
> >   	}
> >   
> >   	/*
> > diff --git a/include/drm/ttm/ttm_bo_api.h b/include/drm/ttm/ttm_bo_api.h
> > index 43c4929a2171..21c7d0d28757 100644
> > --- a/include/drm/ttm/ttm_bo_api.h
> > +++ b/include/drm/ttm/ttm_bo_api.h
> > @@ -155,7 +155,6 @@ struct ttm_tt;
> >    * @offset: The current GPU offset, which can have different meanings
> >    * depending on the memory type. For SYSTEM type memory, it should be 0.
> >    * @cur_placement: Hint of current placement.
> > - * @wu_mutex: Wait unreserved mutex.
> >    *
> >    * Base class for TTM buffer object, that deals with data placement and CPU
> >    * mappings. GPU mappings are really up to the driver, but for simpler GPUs
> > @@ -229,8 +228,6 @@ struct ttm_buffer_object {
> >   	uint64_t offset; /* GPU address space is independent of CPU word size */
> >   
> >   	struct sg_table *sg;
> > -
> > -	struct mutex wu_mutex;
> >   };
> >   
> >   /**
> > @@ -765,7 +762,6 @@ 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
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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