On 8/28/23 13:12, Boris Brezillon wrote:
> On Sun, 27 Aug 2023 20:54:43 +0300
> Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:
>
>> In a preparation of adding drm-shmem memory shrinker, move all reservation
>> locking lockdep checks to use new drm_gem_shmem_resv_assert_held() that
>> will resolve spurious lockdep warning about wrong locking order vs
>> fs_reclam code paths during freeing of shmem GEM, where lockdep isn't
>> aware that it's impossible to have locking contention with the fs_reclam
>> at this special time.
>>
>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
>> ---
>> drivers/gpu/drm/drm_gem_shmem_helper.c | 37 +++++++++++++++++---------
>> 1 file changed, 25 insertions(+), 12 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> index d96fee3d6166..ca5da976aafa 100644
>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> @@ -128,6 +128,23 @@ struct drm_gem_shmem_object *drm_gem_shmem_create(struct drm_device *dev, size_t
>> }
>> EXPORT_SYMBOL_GPL(drm_gem_shmem_create);
>>
>> +static void drm_gem_shmem_resv_assert_held(struct drm_gem_shmem_object *shmem)
>> +{
>> + /*
>> + * Destroying the object is a special case.. drm_gem_shmem_free()
>> + * calls many things that WARN_ON if the obj lock is not held. But
>> + * acquiring the obj lock in drm_gem_shmem_free() can cause a locking
>> + * order inversion between reservation_ww_class_mutex and fs_reclaim.
>> + *
>> + * This deadlock is not actually possible, because no one should
>> + * be already holding the lock when drm_gem_shmem_free() is called.
>> + * Unfortunately lockdep is not aware of this detail. So when the
>> + * refcount drops to zero, we pretend it is already locked.
>> + */
>> + if (kref_read(&shmem->base.refcount))
>> + drm_gem_shmem_resv_assert_held(shmem);
>> +}
>> +
>> /**
>> * drm_gem_shmem_free - Free resources associated with a shmem GEM object
>> * @shmem: shmem GEM object to free
>> @@ -142,8 +159,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>> if (obj->import_attach) {
>> drm_prime_gem_destroy(obj, shmem->sgt);
>> } else if (!shmem->imported_sgt) {
>> - dma_resv_lock(shmem->base.resv, NULL);
>> -
>> drm_WARN_ON(obj->dev, kref_read(&shmem->vmap_use_count));
>>
>> if (shmem->sgt) {
>> @@ -156,8 +171,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>> drm_gem_shmem_put_pages_locked(shmem);
>
> AFAICT, drm_gem_shmem_put_pages_locked() is the only function that's
> called in the free path and would complain about resv-lock not being
> held. I think I'd feel more comfortable if we were adding a
> drm_gem_shmem_free_pages() function that did everything
> drm_gem_shmem_put_pages_locked() does except for the lock_held() check
> and the refcount dec, and have it called here (and in
> drm_gem_shmem_put_pages_locked()). This way we can keep using
> dma_resv_assert_held() instead of having our own variant.
It's not only drm_gem_shmem_free_pages(), but any drm-shmem function
that drivers may use in the GEM's freeing callback.
For example, panfrost_gem_free_object() may unpin shmem BO and then do
drm_gem_shmem_free().
--
Best regards,
Dmitry
