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

Wed Aug 21 14:09:27 UTC 2019

On Wed, Aug 21, 2019 at 2:47 PM Thomas Hellström (VMware)
<thomas_os at shipmail.org> wrote:
> 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 at intel.com>
> >> Cc: Christian Koenig <christian.koenig at amd.com>
> >> Cc: Huang Rui <ray.huang at amd.com>
> >> Cc: Gerd Hoffmann <kraxel at redhat.com>
> >> Cc: "VMware Graphics" <linux-graphics-maintainer at vmware.com>
> >> Cc: Thomas Hellstrom <thellstrom at 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.

That would be patches 1&2 in this series.

> 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.

Christian already noticed that one too, I simply forgot, it's fixed in
v2 I have here.
-Daniel

> /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
> >
> >
>

--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch