Re: [PATCH] drm/i915: Move fd_install after last use of fence

Tvrtko Ursulin
2023-02-06T09:36:04Z

On 03/02/2023 18:15, Rob Clark wrote:
> On Fri, Feb 3, 2023 at 8:49 AM Rob Clark <robdclark@gmail.com> wrote:
>>
>> From: Rob Clark <robdclark@chromium.org>
>>
>> Because eb_composite_fence_create() drops the fence_array reference
>> after creation of the sync_file, only the sync_file holds a ref to the
>> fence. But fd_install() makes that reference visable to userspace, so
>> it must be the last thing we do with the fence.
>>
>
> Fixes: 00dae4d3d35d ("drm/i915: Implement SINGLE_TIMELINE with a syncobj (v4)")

This is correct and the fix looks good to me.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

CI is green so I will merge it, thanks again for a fix Rob!

Followup up question for Matthew Brost however is whether the composite
fence flow could be simplified. This block here comes late in
i915_gem_do_execbuffer and may mislead the user the composite fence is
held to the end of the function:

if (!out_fence && eb.composite_fence)
dma_fence_put(eb.composite_fence);

Question is would it work to remove the !out_fence condition from here,
and remove "consumption" of the reference from eb_composite_fence_create
success path.

Regards,

Tvrtko

>> Signed-off-by: Rob Clark <robdclark@chromium.org>
>> ---
>> drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 14 +++++++-------
>> 1 file changed, 7 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
>> index f266b68cf012..0f2e056c02dd 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
>> @@ -3476,38 +3476,38 @@ i915_gem_do_execbuffer(struct drm_device *dev,
>>
>> err_request:
>> eb_requests_get(&eb);
>> err = eb_requests_add(&eb, err);
>>
>> if (eb.fences)
>> signal_fence_array(&eb, eb.composite_fence ?
>> eb.composite_fence :
>> &eb.requests[0]->fence);
>>
>> + if (unlikely(eb.gem_context->syncobj)) {
>> + drm_syncobj_replace_fence(eb.gem_context->syncobj,
>> + eb.composite_fence ?
>> + eb.composite_fence :
>> + &eb.requests[0]->fence);
>> + }
>> +
>> if (out_fence) {
>> if (err == 0) {
>> fd_install(out_fence_fd, out_fence->file);
>> args->rsvd2 &= GENMASK_ULL(31, 0); /* keep in-fence */
>> args->rsvd2 |= (u64)out_fence_fd << 32;
>> out_fence_fd = -1;
>> } else {
>> fput(out_fence->file);
>> }
>> }
>>
>> - if (unlikely(eb.gem_context->syncobj)) {
>> - drm_syncobj_replace_fence(eb.gem_context->syncobj,
>> - eb.composite_fence ?
>> - eb.composite_fence :
>> - &eb.requests[0]->fence);
>> - }
>> -
>> if (!out_fence && eb.composite_fence)
>> dma_fence_put(eb.composite_fence);
>>
>> eb_requests_put(&eb);
>>
>> err_vma:
>> eb_release_vmas(&eb, true);
>> WARN_ON(err == -EDEADLK);
>> i915_gem_ww_ctx_fini(&eb.ww);
>>
>> --
>> 2.38.1
>>