On 8/11/23 16:08, Steven Price wrote:
> On 09/08/2023 17:53, Boris Brezillon wrote:
>> This way we can grab a pages ref without acquiring the resv lock when
>> pages_use_count > 0. Need to implement asynchronous map using the
> 
> NIT: s/Need/This is needed/
> 
>> drm_gpuva_mgr when the map/unmap operation triggers a mapping split,
>> requiring the new left/right regions to grab an additional page ref
>> to guarantee that the pages stay pinned when the middle section is
>> unmapped.
>>
>> Signed-off-by: Boris Brezillon <boris.brezil...@collabora.com>
>> ---
>>  drivers/gpu/drm/drm_gem_shmem_helper.c  | 28 +++++++++++++------------
>>  drivers/gpu/drm/lima/lima_gem.c         |  2 +-
>>  drivers/gpu/drm/panfrost/panfrost_mmu.c |  2 +-
>>  include/drm/drm_gem_shmem_helper.h      |  2 +-
>>  4 files changed, 18 insertions(+), 16 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c 
>> b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> index a783d2245599..ca6938ea1b82 100644
>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> @@ -155,7 +155,7 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object 
>> *shmem)
>>              if (shmem->pages)
>>                      drm_gem_shmem_put_pages(shmem);
>>  
>> -            drm_WARN_ON(obj->dev, shmem->pages_use_count);
>> +            drm_WARN_ON(obj->dev, atomic_read(&shmem->pages_use_count));
>>  
>>              dma_resv_unlock(shmem->base.resv);
>>      }
>> @@ -172,14 +172,14 @@ static int drm_gem_shmem_get_pages(struct 
>> drm_gem_shmem_object *shmem)
>>  
>>      dma_resv_assert_held(shmem->base.resv);
>>  
>> -    if (shmem->pages_use_count++ > 0)
>> +    if (atomic_inc_return(&shmem->pages_use_count) > 1)
>>              return 0;
>>  
>>      pages = drm_gem_get_pages(obj);
>>      if (IS_ERR(pages)) {
>>              drm_dbg_kms(obj->dev, "Failed to get pages (%ld)\n",
>>                          PTR_ERR(pages));
>> -            shmem->pages_use_count = 0;
>> +            atomic_set(&shmem->pages_use_count, 0);
>>              return PTR_ERR(pages);
>>      }
>>  
>> @@ -210,10 +210,10 @@ void drm_gem_shmem_put_pages(struct 
>> drm_gem_shmem_object *shmem)
>>  
>>      dma_resv_assert_held(shmem->base.resv);
>>  
>> -    if (drm_WARN_ON_ONCE(obj->dev, !shmem->pages_use_count))
>> +    if (drm_WARN_ON_ONCE(obj->dev, !atomic_read(&shmem->pages_use_count)))
>>              return;
>>  
>> -    if (--shmem->pages_use_count > 0)
>> +    if (atomic_dec_return(&shmem->pages_use_count) > 0)
>>              return;
>>  
>>  #ifdef CONFIG_X86
>> @@ -263,6 +263,10 @@ int drm_gem_shmem_pin(struct drm_gem_shmem_object 
>> *shmem)
>>  
>>      drm_WARN_ON(obj->dev, obj->import_attach);
>>  
>> +    /* If we are the first owner, we need to grab the lock. */
>> +    if (atomic_inc_not_zero(&shmem->pages_use_count))
>> +            return 0;
>> +
> 
> Unless I'm misunderstanding I think this introduces a race where two
> threads call drm_gem_shmem_pin() at the same time:
> 
> Thread1                               | Thread 2
> --------------------------------+------------------------------
> drm_gem_shmem_pin()           |
>  - pages_use_count == 0 so not  |
>    incremented                  |
>  - lock taken                 |
> drm_gem_shmem_pin_locked()    |
> drm_gem_shmem_get_pages()     |
>  - pages_use_count incremented        |
> <thread descheduled>            | drm_gem_shmem_pin()
>                                 |  - pages_use_count == 1 so is it
>                               |    incremented and returns early
>                               |    without taking the lock
>                               | Code tries to use shmem->pages
> <thread rescheduled>          | and blows up
> drm_gem_get_pages()           |
> shmem->pages populated                |
> lock released                 |
> 
> I think you need to modify drm_gem_shmem_get_pages() to only increment
> pages_use_count when shmem->pages has been populated. That also gets rid
> of the atomic_set() in that function which scares me.

This is correct, both pin() and get_pages() should use
atomic_inc_not_zero().

Note that we shouldn't use atomic functions open-coded, there is kref
helper for that which uses refcount_t underneath and has additional
checks/warnings for count underflow/overflow. I'm going to post patches
converting drm-shmem to kref around next week, Boris is aware about it
and we should then sync shrinker/panthor patchsets to the common
drm-shmem base.

-- 
Best regards,
Dmitry

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