Messages in this thread | | | Date | Fri, 29 Apr 2022 11:03:26 +0800 | Subject | Re: [PATCH] gpu: drm: remove redundant dma_fence_put() when drm_sched_job_add_dependency() fails | From | Hangyu Hua <> |
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On 2022/4/28 23:27, Andrey Grodzovsky wrote: > > On 2022-04-28 04:56, Hangyu Hua wrote: >> On 2022/4/27 22:43, Andrey Grodzovsky wrote: >>> >>> On 2022-04-26 22:31, Hangyu Hua wrote: >>>> On 2022/4/26 22:55, Andrey Grodzovsky wrote: >>>>> >>>>> On 2022-04-25 22:54, Hangyu Hua wrote: >>>>>> On 2022/4/25 23:42, Andrey Grodzovsky wrote: >>>>>>> On 2022-04-25 04:36, Hangyu Hua wrote: >>>>>>> >>>>>>>> When drm_sched_job_add_dependency() fails, dma_fence_put() will >>>>>>>> be called >>>>>>>> internally. Calling it again after >>>>>>>> drm_sched_job_add_dependency() finishes >>>>>>>> may result in a dangling pointer. >>>>>>>> >>>>>>>> Fix this by removing redundant dma_fence_put(). >>>>>>>> >>>>>>>> Signed-off-by: Hangyu Hua <hbh25y@gmail.com> >>>>>>>> --- >>>>>>>> drivers/gpu/drm/lima/lima_gem.c | 1 - >>>>>>>> drivers/gpu/drm/scheduler/sched_main.c | 1 - >>>>>>>> 2 files changed, 2 deletions(-) >>>>>>>> >>>>>>>> diff --git a/drivers/gpu/drm/lima/lima_gem.c >>>>>>>> b/drivers/gpu/drm/lima/lima_gem.c >>>>>>>> index 55bb1ec3c4f7..99c8e7f6bb1c 100644 >>>>>>>> --- a/drivers/gpu/drm/lima/lima_gem.c >>>>>>>> +++ b/drivers/gpu/drm/lima/lima_gem.c >>>>>>>> @@ -291,7 +291,6 @@ static int lima_gem_add_deps(struct drm_file >>>>>>>> *file, struct lima_submit *submit) >>>>>>>> err = >>>>>>>> drm_sched_job_add_dependency(&submit->task->base, fence); >>>>>>>> if (err) { >>>>>>>> - dma_fence_put(fence); >>>>>>>> return err; >>>>>>> >>>>>>> >>>>>>> Makes sense here >>>>>>> >>>>>>> >>>>>>>> } >>>>>>>> } >>>>>>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c >>>>>>>> b/drivers/gpu/drm/scheduler/sched_main.c >>>>>>>> index b81fceb0b8a2..ebab9eca37a8 100644 >>>>>>>> --- a/drivers/gpu/drm/scheduler/sched_main.c >>>>>>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c >>>>>>>> @@ -708,7 +708,6 @@ int >>>>>>>> drm_sched_job_add_implicit_dependencies(struct drm_sched_job *job, >>>>>>>> dma_fence_get(fence); >>>>>>>> ret = drm_sched_job_add_dependency(job, fence); >>>>>>>> if (ret) { >>>>>>>> - dma_fence_put(fence); >>>>>>> >>>>>>> >>>>>>> >>>>>>> Not sure about this one since if you look at the relevant commits - >>>>>>> 'drm/scheduler: fix drm_sched_job_add_implicit_dependencies' and >>>>>>> 'drm/scheduler: fix drm_sched_job_add_implicit_dependencies harder' >>>>>>> You will see that the dma_fence_put here balances the extra >>>>>>> dma_fence_get >>>>>>> above >>>>>>> >>>>>>> Andrey >>>>>>> >>>>>> >>>>>> I don't think so. I checked the call chain and found no additional >>>>>> dma_fence_get(). But dma_fence_get() needs to be called before >>>>>> drm_sched_job_add_dependency() to keep the counter balanced. >>>>> >>>>> >>>>> I don't say there is an additional get, I just say that >>>>> drm_sched_job_add_dependency doesn't grab an extra reference to the >>>>> fences it stores so this needs to be done outside and for that >>>>> drm_sched_job_add_implicit_dependencies->dma_fence_get is called >>>>> and, if this addition fails you just call dma_fence_put to keep the >>>>> counter balanced. >>>>> >>>> >>>> drm_sched_job_add_implicit_dependencies() will call >>>> drm_sched_job_add_dependency(). And drm_sched_job_add_dependency() >>>> already call dma_fence_put() when it fails. Calling dma_fence_put() >>>> twice doesn't make sense. >>>> >>>> dma_fence_get() is in [2]. But dma_fence_put() will be called in [1] >>>> and [3] when xa_alloc() fails. >>> >>> >>> The way I see it, [2] and [3] are mat matching *get* and *put* >>> respectively. [1] *put* is against the original >>> dma_fence_init->kref_init of the fence which always set the refcount >>> at 1. >>> Also in support of this see commit 'drm/scheduler: fix >>> drm_sched_job_add_implicit_dependencies harder' - it says there >>> "drm_sched_job_add_dependency() could drop the last ref" - this last >>> ref is the original refcount set by dma_fence_init->kref >>> >>> Andrey >> >> >> You can see that drm_sched_job_add_dependency() has three return paths >> they are [4], [5] and [1]. [4] and [5] will return 0. [1] will return >> error. >> >> There will be three weird problems if you're right: >> >> 1. [5] path will triger a refcount leak beacause ret is 0 in *if*[6]. > > > Terminology confusion issue - [5] is a 'put' so it cannot cause a leak > by definition, extra unbalanced 'get' will cause a leak because memory > is never released, extra put will just probably cause a warning in > kref_put or maybe double free. > > >> Otherwise [2] and [5] are matching *get* and *put* in here. > > > Exactly, they are matching - so until this point all good and no 'leak' > then, no ? >
In fact, i just want to prove that [2] and [3] are not a matching pair when the path go [4] or [5]. It's less likely when the path is [1]. But it doesn't matter, please see my explanation below.
> >> >> 2. [4] path need a additional dma_fence_get() to adds the fence as a >> job dependency. fence is from obj->resv. Taking msm as an example >> obj->resv is from etnaviv_ioctl_gem_submit()->submit_lookup_objects(). >> It is not possible that an object has *refcount == 1* but is >> referenced in two places. So dma_fence_get() called in [2] is for [4]. >> By the way, [3] don't execute in this case. > > > Still don't see the problem - [2] is the additional dma_fence_get() you > need here (just as you say above). > > >> >> 3. This one is a doubt. You can see in "[PATCH] drm/scheduler: fix >> drm_sched_job_add_implicit_dependencies harder". >> drm_sched_job_add_dependency() could drop the last ref, so we need to do >> the dma_fence_get() first. But the last ref still will drop in [3] if >> drm_sched_job_add_dependency() go path [1]. And there is only a >> *return* between [1] and [3]. Is this necessary? I think Rob Clark >> wants to avoid the last ref being dropped in >> drm_sched_job_add_implicit_dependencies() because fence is still used >> by obj->resv. > > > In the scenario above - if we go thorough path [1] refcount before [1] > starts is 2 - one from original kref_init and one from [2] and so it's > balanced against 2 puts (one from [1] and one from [3]) so I still don't > see a problem. >
We can't directly drop the last refcount and release fence in drm_sched_job_add_implicit_dependencies. fence is from obj->resv. Taking msm as an example obj->resv is from msm_ioctl_gem_submit()->submit_lookup_objects().
static int submit_lookup_objects(struct msm_gem_submit *submit, struct drm_msm_gem_submit *args, struct drm_file *file) { ... for (i = 0; i < args->nr_bos; i++) { struct drm_gem_object *obj;
/* normally use drm_gem_object_lookup(), but for bulk lookup * all under single table_lock just hit object_idr directly: */ obj = idr_find(&file->object_idr, submit->bos[i].handle); <---- we find obj in here by a user controllable handle if (!obj) { DRM_ERROR("invalid handle %u at index %u\n", submit->bos[i].handle, i); ret = -EINVAL; goto out_unlock; }
drm_gem_object_get(obj);
submit->bos[i].obj = to_msm_bo(obj); <---- we store it } ... }
Taking msm as an example, the patch to call drm_sched_job_add_implicit_dependencies() is msm_ioctl_gem_submit()->submit_fence_sync().
static int submit_fence_sync(struct msm_gem_submit *submit, bool no_implicit) { int i, ret = 0;
for (i = 0; i < submit->nr_bos; i++) { struct drm_gem_object *obj = &submit->bos[i].obj->base; <---- get the obj ... ret = drm_sched_job_add_implicit_dependencies(&submit->base, obj, write); if (ret) break; }
return ret; }
If fence is released in drm_sched_job_add_implicit_dependencies(), a dangling pointer will be in obj->resv.
specific scenario: recount = 1 init, obj->resv->fence_excl = fence recount = 1 before drm_sched_job_add_implicit_dependencies recount = 2 in [2] recount = 1 in [1] recount = 0 in [3] <--- fence release. But fence still in obj->resv
Thanks, Hangyu
> I suggest that you give a specific scenario from fence ref-count > perspective that your patch fixes. I might be wrong but unless you give > a specific case where the 'put' in [3] is redundant I just can't see it. > > Andrey > > >> >> >> int drm_sched_job_add_dependency(struct drm_sched_job *job, >> struct dma_fence *fence) >> { >> ... >> xa_for_each(&job->dependencies, index, entry) { >> if (entry->context != fence->context) >> continue; >> >> if (dma_fence_is_later(fence, entry)) { >> dma_fence_put(entry); >> xa_store(&job->dependencies, index, fence, >> GFP_KERNEL); <---- [4] >> } else { >> dma_fence_put(fence); <---- [5] >> } >> return 0; >> } >> >> ret = xa_alloc(&job->dependencies, &id, fence, xa_limit_32b, >> GFP_KERNEL); >> if (ret != 0) >> dma_fence_put(fence); <---- [1] >> >> return ret; >> } >> >> >> int drm_sched_job_add_implicit_dependencies(struct drm_sched_job *job, >> struct drm_gem_object *obj, >> bool write) >> { >> struct dma_resv_iter cursor; >> struct dma_fence *fence; >> int ret; >> >> dma_resv_for_each_fence(&cursor, obj->resv, write, fence) { >> /* Make sure to grab an additional ref on the added >> fence */ >> dma_fence_get(fence); <---- [2] >> ret = drm_sched_job_add_dependency(job, fence); >> if (ret) { <---- [6] >> dma_fence_put(fence); <---- [3] >> >> return ret; >> } >> } >> return 0; >> } >> >> Thanks, >> hangyu >> >>> >>>> >>>> >>>> int drm_sched_job_add_dependency(struct drm_sched_job *job, >>>> struct dma_fence *fence) >>>> { >>>> ... >>>> ret = xa_alloc(&job->dependencies, &id, fence, xa_limit_32b, >>>> GFP_KERNEL); >>>> if (ret != 0) >>>> dma_fence_put(fence); <--- [1] >>>> >>>> return ret; >>>> } >>>> EXPORT_SYMBOL(drm_sched_job_add_dependency); >>>> >>>> >>>> int drm_sched_job_add_implicit_dependencies(struct drm_sched_job *job, >>>> struct drm_gem_object *obj, >>>> bool write) >>>> { >>>> struct dma_resv_iter cursor; >>>> struct dma_fence *fence; >>>> int ret; >>>> >>>> dma_resv_for_each_fence(&cursor, obj->resv, write, fence) { >>>> /* Make sure to grab an additional ref on the added fence */ >>>> dma_fence_get(fence); <--- [2] >>>> ret = drm_sched_job_add_dependency(job, fence); >>>> if (ret) { >>>> dma_fence_put(fence); <--- [3] >>>> return ret; >>>> } >>>> } >>>> return 0; >>>> } >>>> >>>> >>>>> >>>>>> On the other hand, dma_fence_get() and dma_fence_put() are >>>>>> meaningless here if threre is an extra dma_fence_get() beacause >>>>>> counter will not decrease to 0 during drm_sched_job_add_dependency(). >>>>>> >>>>>> I check the call chain as follows: >>>>>> >>>>>> msm_ioctl_gem_submit() >>>>>> -> submit_fence_sync() >>>>>> -> drm_sched_job_add_implicit_dependencies() >>>>> >>>>> >>>>> Can you maybe trace or print one such example of problematic >>>>> refcount that you are trying to fix ? I still don't see where is >>>>> the problem. >>>>> >>>>> Andrey >>>>> >>>> >>>> I also wish I could. System logs can make this easy. But i don't >>>> have a corresponding GPU physical device. >>>> drm_sched_job_add_implicit_dependencies is only used in a few devices. >>>> >>>> Thanks. >>>>> >>>>>> >>>>>> Thanks, >>>>>> Hangyu >>>>>> >>>>>>> >>>>>>>> return ret; >>>>>>>> } >>>>>>>> }
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