Re: [RFC 26/29] dma-buf/fence: remove pointless fence_timeline_signal at destroy phase

From: John Harrison
Date: Fri Jan 15 2016 - 12:48:57 EST

On 15/01/2016 14:55, Gustavo Padovan wrote:
From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxxxx>

All changes to timeline value come through the user via
fence_timeline_signal() calls. When fence_timeline_destroy() is called no
changes on timeline->value happens hence call fence_timeline_signal() with
no increment is pointless.

Signed-off-by: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxxxx>
drivers/dma-buf/fence.c | 6 +-----
1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/drivers/dma-buf/fence.c b/drivers/dma-buf/fence.c
index 7a5fc9b..26f5f0f 100644
--- a/drivers/dma-buf/fence.c
+++ b/drivers/dma-buf/fence.c
@@ -136,7 +136,7 @@ EXPORT_SYMBOL(fence_timeline_put);
* fence_timeline_destroy - destroy a fence_timeline
* @timeline [in] the fence_timeline to destroy
- * This function destroys a timeline. It signals any active fence first.
+ * This function destroys a timeline.

The implementation for this was certainly broken but I would say it should be fixed to match the comment rather than just abandoned completely. That is, what happens if a timeline owner destroys their timeline while there are outstanding fences which other drivers are waiting on? That is presumably a bug in the code that called destroy prematurely, but bugs happen.

The old implementation simply leaked the fences. Doing a debugfs dump would show the timeline with all its outstanding fences still floating around forever after. Worse, anything waiting on them would never be signalled and is therefore potentially deadlocked.

Note that I haven't had chance to look through the entire patch series yet so maybe this has been fixed up elsewhere. If not, then I think it definitely needs looking into.

void fence_timeline_destroy(struct fence_timeline *timeline)
@@ -147,10 +147,6 @@ void fence_timeline_destroy(struct fence_timeline *timeline)
- /*
- * signal any children that their parent is going away.
- */
- fence_timeline_signal(timeline, 0);