2023-02-03 16:50:02

by Rob Clark

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

From: Rob Clark <[email protected]>

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.

Signed-off-by: Rob Clark <[email protected]>
---
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

2023-02-03 18:16:02

by Rob Clark

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

On Fri, Feb 3, 2023 at 8:49 AM Rob Clark <[email protected]> wrote:
>
> From: Rob Clark <[email protected]>
>
> 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)")

> Signed-off-by: Rob Clark <[email protected]>
> ---
> 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
>

2023-02-06 09:36:04

by Tvrtko Ursulin

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

On 03/02/2023 18:15, Rob Clark wrote:
> On Fri, Feb 3, 2023 at 8:49 AM Rob Clark <[email protected]> wrote:
>>
>> From: Rob Clark <[email protected]>
>>
>> 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 <[email protected]>

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 <[email protected]>
>> ---
>> 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
>>