Re: [PATCH 1/2] dma-buf/sw_sync: Avoid recursive lock during fence signal

[Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>]

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>

On Wed, Jul 15, 2020 at 12:04 PM Chris Wilson <chris@chris-wilson.co.uk> wrote:
>
> If a signal callback releases the sw_sync fence, that will trigger a
> deadlock as the timeline_fence_release recurses onto the fence->lock
> (used both for signaling and the the timeline tree).
>
> If we always hold a reference for an unsignaled fence held by the
> timeline, we no longer need to detach the fence from the timeline upon
> release. This is only possible since commit ea4d5a270b57
> ("dma-buf/sw_sync: force signal all unsignaled fences on dying timeline")
> where we introduced decoupling of the fences from the timeline upon release.
>
> Reported-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
> Fixes: d3c6dd1fb30d ("dma-buf/sw_sync: Synchronize signal vs syncpt free")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Sumit Semwal <sumit.semwal@linaro.org>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Gustavo Padovan <gustavo@padovan.org>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: <stable@vger.kernel.org>
> ---
>  drivers/dma-buf/sw_sync.c | 32 +++++++-------------------------
>  1 file changed, 7 insertions(+), 25 deletions(-)
>
> diff --git a/drivers/dma-buf/sw_sync.c b/drivers/dma-buf/sw_sync.c
> index 348b3a9170fa..4cc2ac03a84a 100644
> --- a/drivers/dma-buf/sw_sync.c
> +++ b/drivers/dma-buf/sw_sync.c
> @@ -130,16 +130,7 @@ static const char *timeline_fence_get_timeline_name(struct dma_fence *fence)
>
>  static void timeline_fence_release(struct dma_fence *fence)
>  {
> -       struct sync_pt *pt = dma_fence_to_sync_pt(fence);
>         struct sync_timeline *parent = dma_fence_parent(fence);
> -       unsigned long flags;
> -
> -       spin_lock_irqsave(fence->lock, flags);
> -       if (!list_empty(&pt->link)) {
> -               list_del(&pt->link);
> -               rb_erase(&pt->node, &parent->pt_tree);
> -       }
> -       spin_unlock_irqrestore(fence->lock, flags);
>
>         sync_timeline_put(parent);
>         dma_fence_free(fence);
> @@ -203,18 +194,11 @@ static void sync_timeline_signal(struct sync_timeline *obj, unsigned int inc)
>                 if (!timeline_fence_signaled(&pt->base))
>                         break;
>
> -               list_del_init(&pt->link);
> +               list_del(&pt->link);
>                 rb_erase(&pt->node, &obj->pt_tree);
>
> -               /*
> -                * A signal callback may release the last reference to this
> -                * fence, causing it to be freed. That operation has to be
> -                * last to avoid a use after free inside this loop, and must
> -                * be after we remove the fence from the timeline in order to
> -                * prevent deadlocking on timeline->lock inside
> -                * timeline_fence_release().
> -                */
>                 dma_fence_signal_locked(&pt->base);
> +               dma_fence_put(&pt->base);
>         }
>
>         spin_unlock_irq(&obj->lock);
> @@ -261,13 +245,9 @@ static struct sync_pt *sync_pt_create(struct sync_timeline *obj,
>                         } else if (cmp < 0) {
>                                 p = &parent->rb_left;
>                         } else {
> -                               if (dma_fence_get_rcu(&other->base)) {
> -                                       sync_timeline_put(obj);
> -                                       kfree(pt);
> -                                       pt = other;
> -                                       goto unlock;
> -                               }
> -                               p = &parent->rb_left;
> +                               dma_fence_put(&pt->base);
> +                               pt = other;
> +                               goto unlock;
>                         }
>                 }
>                 rb_link_node(&pt->node, parent, p);
> @@ -278,6 +258,7 @@ static struct sync_pt *sync_pt_create(struct sync_timeline *obj,
>                               parent ? &rb_entry(parent, typeof(*pt), node)->link : &obj->pt_list);
>         }
>  unlock:
> +       dma_fence_get(&pt->base); /* keep a ref for the timeline */
>         spin_unlock_irq(&obj->lock);
>
>         return pt;
> @@ -316,6 +297,7 @@ static int sw_sync_debugfs_release(struct inode *inode, struct file *file)
>         list_for_each_entry_safe(pt, next, &obj->pt_list, link) {
>                 dma_fence_set_error(&pt->base, -ENOENT);
>                 dma_fence_signal_locked(&pt->base);
> +               dma_fence_put(&pt->base);
>         }
>
>         spin_unlock_irq(&obj->lock);
> --
> 2.20.1
>
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