On 23/11/2022 09:28, Janusz Krzysztofik wrote:
Hi Tvrtko,
Thanks for your comments.
On Tuesday, 22 November 2022 11:50:38 CET Tvrtko Ursulin wrote:
On 21/11/2022 14:56, Janusz Krzysztofik wrote:
Users of intel_gt_retire_requests_timeout() expect 0 return value on
success. However, we have no protection from passing back 0 potentially
returned by a call to dma_fence_wait_timeout() when it succedes right
after its timeout has expired.
Is this talking about a potential weakness, or ambiguous kerneldoc, of
dma_fence_wait_timeout, dma_fence_default_wait and
i915_request_wait_timeout? They appear to say 0 return means timeout,
implying unsignaled fence. In other words signaled must return positive
remaining timeout. Implementations seems to allow a race which indeed
appears that return 0 and signaled fence is possible.
While my initial analysis was indeed focused on inconsistent semantics of 0
return values from different dma_fence_default_wait() backends, I should have
also mentioned in this commit description that users may perfectly
call intel_gt_retire_requests_timeout() with 0 timeout, in which case the
false positive 0 value can be returned regardless of dma_fence_wait_timeout()
potential issues. Would you like me to reword and resubmit?
Not sure yet.
So the only caller which passes in zero to
intel_gt_retire_requests_timeout appears to be intel_gt_retire_requests
and it eats the return value anyway so this patch is immaterial for that
one.
I guess it can change how intel_gt_wait_for_idle behaves with short-ish
timeouts. In case this race is hit. But then wouldn't it make sense to
follow up with a patch which addresses this race by re-checking the "is
signaled" when timeout expires, either in dma_fence_wait_timeout, or to
intel_gt_retire_requests_timeout. Or if not that at least better
document the dma_fence_wait_timeout and/or
intel_gt_retire_requests_timeout. Makes sense?
Regards,
Tvrtko
If dma_fence_wait can indeed return 0 even when a request is signaled,
then how is timeout ?: -ETIME below correct? It isn't a chance for false
negative in its' callers?
The goal of intel_gt_retire_requests_timeout() is to retire requests. When
that goal has been reached, i.e., all requests have been retired, active count
is 0, and 0 is correctly returned, regardless of timeout value.
The value of timeout is used only when there are still pending requests, which
means that the goal hasn't been reached and the function hasn't succeeded.
Then, no false negative is possible, unlike the false positive that we now
have when we return 0 while some requests are still pending.
Thanks,
Janusz
Regards,
Tvrtko
Replace 0 with -ETIME before potentially using the timeout value as return
code, so -ETIME is returned if there are still some requests not retired
after timeout, 0 otherwise.
v3: Use conditional expression, more compact but also better reflecting
intention standing behind the change.
v2: Move the added lines down so flush_submission() is not affected.
Fixes: f33a8a51602c ("drm/i915: Merge wait_for_timelines with retire_request")
Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@xxxxxxxxxxxxxxx>
Reviewed-by: Andrzej Hajda <andrzej.hajda@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v5.5+
---
drivers/gpu/drm/i915/gt/intel_gt_requests.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/i915/gt/intel_gt_requests.c b/drivers/gpu/drm/i915/gt/intel_gt_requests.c
index edb881d756309..1dfd01668c79c 100644
--- a/drivers/gpu/drm/i915/gt/intel_gt_requests.c
+++ b/drivers/gpu/drm/i915/gt/intel_gt_requests.c
@@ -199,7 +199,7 @@ out_active: spin_lock(&timelines->lock);
if (remaining_timeout)
*remaining_timeout = timeout;
- return active_count ? timeout : 0;
+ return active_count ? timeout ?: -ETIME : 0;
}
static void retire_work_handler(struct work_struct *work)
