Re: [Intel-gfx] [PATCH v15 17/23] drm/shmem-helper: Add and use drm_gem_shmem_resv_assert_held() helper

2023-09-04 Thread Boris Brezillon 
On Sat, 2 Sep 2023 22:43:02 +0300
Dmitry Osipenko  wrote:

> On 8/29/23 10:29, Boris Brezillon wrote:
> > On Tue, 29 Aug 2023 05:34:23 +0300
> > Dmitry Osipenko  wrote:
> >   
> >> On 8/28/23 13:12, Boris Brezillon wrote:  
> >>> On Sun, 27 Aug 2023 20:54:43 +0300
> >>> Dmitry Osipenko  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 
>  ---
>   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(>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(>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().  
> > 
> > Is this really a valid use case? If the GEM refcount dropped to zero,
> > we should certainly not have pages_pin_count > 0 (thinking of vmap-ed
> > buffers that might disappear while kernel still has a pointer to the
> > CPU-mapped area). The only reason we have this
> > drm_gem_shmem_put_pages_locked() in drm_gem_shmem_free() is because of
> > this implicit ref hold by the sgt, and IMHO, we should be stricter and
> > check that pages_use_count == 1 when sgt != NULL and pages_use_count ==
> > 0 otherwise.
> > 
> > I actually think it's a good thing to try and catch any attempt to call
> > functions trying lock the resv in a path they're not supposed to. At
> > least we can decide whether these actions are valid or not in this
> > context, and provide dedicated helpers for the free path if they are.  
> 
> To me it's a valid use-case. I was going to do it for the virtio-gpu
> driver for a specific BO type that should be permanently pinned in
> memory. So I made the BO pinned in the virto_gpu's bo_create() and
>

Re: [Intel-gfx] [PATCH v15 17/23] drm/shmem-helper: Add and use drm_gem_shmem_resv_assert_held() helper

2023-09-02 Thread Dmitry Osipenko 
On 8/29/23 10:29, Boris Brezillon wrote:
> On Tue, 29 Aug 2023 05:34:23 +0300
> Dmitry Osipenko  wrote:
> 
>> On 8/28/23 13:12, Boris Brezillon wrote:
>>> On Sun, 27 Aug 2023 20:54:43 +0300
>>> Dmitry Osipenko  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 
 ---
  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(>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(>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().
> 
> Is this really a valid use case? If the GEM refcount dropped to zero,
> we should certainly not have pages_pin_count > 0 (thinking of vmap-ed
> buffers that might disappear while kernel still has a pointer to the
> CPU-mapped area). The only reason we have this
> drm_gem_shmem_put_pages_locked() in drm_gem_shmem_free() is because of
> this implicit ref hold by the sgt, and IMHO, we should be stricter and
> check that pages_use_count == 1 when sgt != NULL and pages_use_count ==
> 0 otherwise.
> 
> I actually think it's a good thing to try and catch any attempt to call
> functions trying lock the resv in a path they're not supposed to. At
> least we can decide whether these actions are valid or not in this
> context, and provide dedicated helpers for the free path if they are.

To me it's a valid use-case. I was going to do it for the virtio-gpu
driver for a specific BO type that should be permanently pinned in
memory. So I made the BO pinned in the virto_gpu's bo_create() and
unpinned it from the virtio-gpu's gem->free(), this is a perfectly valid
case to me. Though, in the end I switched to another approach that
doesn't require to do the pinning in the virtio-gpu driver.

For now we can do it as you suggested, to use custom put_pages() in the
shmem_free() since neither of drivers need that. Let's try that.

-- 
Best regards,
Dmitry

Re: [Intel-gfx] [PATCH v15 17/23] drm/shmem-helper: Add and use drm_gem_shmem_resv_assert_held() helper

2023-08-29 Thread Boris Brezillon 
On Tue, 29 Aug 2023 11:44:13 +0200
Boris Brezillon  wrote:

> On Tue, 29 Aug 2023 10:52:03 +0200
> Christian König  wrote:
> 
> > Am 29.08.23 um 09:29 schrieb Boris Brezillon:  
> > > On Tue, 29 Aug 2023 05:34:23 +0300
> > > Dmitry Osipenko  wrote:
> > >
> > >> On 8/28/23 13:12, Boris Brezillon wrote:
> > >>> On Sun, 27 Aug 2023 20:54:43 +0300
> > >>> Dmitry Osipenko  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 
> >  ---
> >    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(>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(>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().
> > > Is this really a valid use case?
> > 
> > I haven't followed the whole discussion, but I think it isn't a valid 
> > use case.
> > 
> > That page_use_count is none zero while the GEM object is about to be 
> > destroyed can only happen is someone managed to grab a reference to the 
> > page without referencing the GEM object.  
> 
> Actually, drm_gem_shmem_object is a bit special (weird?) in this regard.
> drm_gem_shmem_get_pages_sgt_locked() creates the sgt and takes a
> pages ref (pages_use_count++). The sgt itself is cached (next call to
> drm_gem_shmem_get_pages_sgt_locked() will return the existing sgt) but
> not refcounted, which means it will stay around until the GEM object is
> destroyed or its pages are purged (GEM eviction). Because of that,
> shmem->pages_use_count == 1 in

Re: [Intel-gfx] [PATCH v15 17/23] drm/shmem-helper: Add and use drm_gem_shmem_resv_assert_held() helper

2023-08-29 Thread Boris Brezillon 
On Tue, 29 Aug 2023 10:52:03 +0200
Christian König  wrote:

> Am 29.08.23 um 09:29 schrieb Boris Brezillon:
> > On Tue, 29 Aug 2023 05:34:23 +0300
> > Dmitry Osipenko  wrote:
> >  
> >> On 8/28/23 13:12, Boris Brezillon wrote:  
> >>> On Sun, 27 Aug 2023 20:54:43 +0300
> >>> Dmitry Osipenko  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 
>  ---
>    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(>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(>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().  
> > Is this really a valid use case?  
> 
> I haven't followed the whole discussion, but I think it isn't a valid 
> use case.
> 
> That page_use_count is none zero while the GEM object is about to be 
> destroyed can only happen is someone managed to grab a reference to the 
> page without referencing the GEM object.

Actually, drm_gem_shmem_object is a bit special (weird?) in this regard.
drm_gem_shmem_get_pages_sgt_locked() creates the sgt and takes a
pages ref (pages_use_count++). The sgt itself is cached (next call to
drm_gem_shmem_get_pages_sgt_locked() will return the existing sgt) but
not refcounted, which means it will stay around until the GEM object is
destroyed or its pages are purged (GEM eviction). Because of that,
shmem->pages_use_count == 1 in drm_gem_shmem_free_pages() is valid iff
shmem->sgt != NULL. pages_use_count > 1 is invalid though, as should be
pages_pin_count after Dmitry's patches.

If we want to 'fix' that (not convinced this is a bug, more a design
choice), we need to refcount the sgt users and

Re: [Intel-gfx] [PATCH v15 17/23] drm/shmem-helper: Add and use drm_gem_shmem_resv_assert_held() helper

2023-08-29 Thread Christian König 

Am 29.08.23 um 09:29 schrieb Boris Brezillon:

On Tue, 29 Aug 2023 05:34:23 +0300
Dmitry Osipenko  wrote:


On 8/28/23 13:12, Boris Brezillon wrote:

On Sun, 27 Aug 2023 20:54:43 +0300
Dmitry Osipenko  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 
---
  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(>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(>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().

Is this really a valid use case?


I haven't followed the whole discussion, but I think it isn't a valid 
use case.


That page_use_count is none zero while the GEM object is about to be 
destroyed can only happen is someone managed to grab a reference to the 
page without referencing the GEM object.


This is turn usually happens when somebody incorrectly walks the CPU 
page tables and grabs page references where it shouldn't. KMS used to do 
this and we had already had a discussion that they shouldn't do this.


Regards,
Christian.



  If the GEM refcount dropped to zero,
we should certainly not have pages_pin_count > 0 (thinking of vmap-ed
buffers that might disappear while kernel still has a pointer to the
CPU-mapped area). The only reason we have this
drm_gem_shmem_put_pages_locked() in drm_gem_shmem_free() is because of
this implicit ref hold by the sgt, and IMHO, we should be stricter and
check that pages_use_count == 1 when sgt != NULL and pages_use_count ==
0 otherwise.

I actually think it's a good thing to try and catch any attempt to call
functions trying lock the resv in a path they're not supposed to. At
least we can decide whether these actions are valid or not in this
context, and provide dedicated helpers for the free path if they are.

Re: [Intel-gfx] [PATCH v15 17/23] drm/shmem-helper: Add and use drm_gem_shmem_resv_assert_held() helper

2023-08-29 Thread Boris Brezillon 
On Tue, 29 Aug 2023 05:34:23 +0300
Dmitry Osipenko  wrote:

> On 8/28/23 13:12, Boris Brezillon wrote:
> > On Sun, 27 Aug 2023 20:54:43 +0300
> > Dmitry Osipenko  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 
> >> ---
> >>  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(>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(>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().

Is this really a valid use case? If the GEM refcount dropped to zero,
we should certainly not have pages_pin_count > 0 (thinking of vmap-ed
buffers that might disappear while kernel still has a pointer to the
CPU-mapped area). The only reason we have this
drm_gem_shmem_put_pages_locked() in drm_gem_shmem_free() is because of
this implicit ref hold by the sgt, and IMHO, we should be stricter and
check that pages_use_count == 1 when sgt != NULL and pages_use_count ==
0 otherwise.

I actually think it's a good thing to try and catch any attempt to call
functions trying lock the resv in a path they're not supposed to. At
least we can decide whether these actions are valid or not in this
context, and provide dedicated helpers for the free path if they are.

Re: [Intel-gfx] [PATCH v15 17/23] drm/shmem-helper: Add and use drm_gem_shmem_resv_assert_held() helper

2023-08-28 Thread Dmitry Osipenko 
On 8/28/23 13:12, Boris Brezillon wrote:
> On Sun, 27 Aug 2023 20:54:43 +0300
> Dmitry Osipenko  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 
>> ---
>>  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(>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(>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

Re: [Intel-gfx] [PATCH v15 17/23] drm/shmem-helper: Add and use drm_gem_shmem_resv_assert_held() helper

2023-08-28 Thread Boris Brezillon 
On Sun, 27 Aug 2023 20:54:43 +0300
Dmitry Osipenko  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 
> ---
>  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(>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(>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.

>  
>   drm_WARN_ON(obj->dev, kref_read(>pages_use_count));
> -
> - dma_resv_unlock(shmem->base.resv);
>   }
>  
>   drm_gem_object_release(obj);
> @@ -170,7 +183,7 @@ static int drm_gem_shmem_get_pages_locked(struct 
> drm_gem_shmem_object *shmem)
>   struct drm_gem_object *obj = >base;
>   struct page **pages;
>  
> - dma_resv_assert_held(shmem->base.resv);
> + drm_gem_shmem_resv_assert_held(shmem);
>  
>   if (kref_get_unless_zero(>pages_use_count))
>   return 0;
> @@ -228,7 +241,7 @@ static void drm_gem_shmem_kref_release_pages(struct kref 
> *kref)
>   */
>  void drm_gem_shmem_put_pages_locked(struct drm_gem_shmem_object *shmem)
>  {
> - dma_resv_assert_held(shmem->base.resv);
> + drm_gem_shmem_resv_assert_held(shmem);
>  
>   kref_put(>pages_use_count, drm_gem_shmem_kref_release_pages);
>  }
> @@ -252,7 +265,7 @@ static int drm_gem_shmem_pin_locked(struct 
> drm_gem_shmem_object *shmem)
>  {
>   int ret;
>  
> - dma_resv_assert_held(shmem->base.resv);
> + drm_gem_shmem_resv_assert_held(shmem);
>  
>   if (kref_get_unless_zero(>pages_pin_count))
>   return 0;
> @@ -276,7 +289,7 @@ static void drm_gem_shmem_kref_unpin_pages(struct kref 
> *kref)
>  
>  static void drm_gem_shmem_unpin_locked(struct drm_gem_shmem_object *shmem)
>  {
> - dma_resv_assert_held(shmem->base.resv);
> + drm_gem_shmem_resv_assert_held(shmem);
>  
>   kref_put(>pages_pin_count, drm_gem_shmem_kref_unpin_pages);
>  }
> @@ -357,7 +370,7 @@ int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object 
> *shmem,
>   } else {
>   pgprot_t prot = PAGE_KERNEL;
>  
> - dma_resv_assert_held(shmem->base.resv);
> + drm_gem_shmem_resv_assert_held(shmem);
>  
>   if (kref_get_unless_zero(>vmap_use_count)) {
>   iosys_map_set_vaddr(map, shmem->vaddr);
> @@ -426,7 +439,7 @@ void drm_gem_shmem_vunmap_locked(struct 
>

[Intel-gfx] [PATCH v15 17/23] drm/shmem-helper: Add and use drm_gem_shmem_resv_assert_held() helper

2023-08-27 Thread Dmitry Osipenko 
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 
---
 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(>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(>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);
 
drm_WARN_ON(obj->dev, kref_read(>pages_use_count));
-
-   dma_resv_unlock(shmem->base.resv);
}
 
drm_gem_object_release(obj);
@@ -170,7 +183,7 @@ static int drm_gem_shmem_get_pages_locked(struct 
drm_gem_shmem_object *shmem)
struct drm_gem_object *obj = >base;
struct page **pages;
 
-   dma_resv_assert_held(shmem->base.resv);
+   drm_gem_shmem_resv_assert_held(shmem);
 
if (kref_get_unless_zero(>pages_use_count))
return 0;
@@ -228,7 +241,7 @@ static void drm_gem_shmem_kref_release_pages(struct kref 
*kref)
  */
 void drm_gem_shmem_put_pages_locked(struct drm_gem_shmem_object *shmem)
 {
-   dma_resv_assert_held(shmem->base.resv);
+   drm_gem_shmem_resv_assert_held(shmem);
 
kref_put(>pages_use_count, drm_gem_shmem_kref_release_pages);
 }
@@ -252,7 +265,7 @@ static int drm_gem_shmem_pin_locked(struct 
drm_gem_shmem_object *shmem)
 {
int ret;
 
-   dma_resv_assert_held(shmem->base.resv);
+   drm_gem_shmem_resv_assert_held(shmem);
 
if (kref_get_unless_zero(>pages_pin_count))
return 0;
@@ -276,7 +289,7 @@ static void drm_gem_shmem_kref_unpin_pages(struct kref 
*kref)
 
 static void drm_gem_shmem_unpin_locked(struct drm_gem_shmem_object *shmem)
 {
-   dma_resv_assert_held(shmem->base.resv);
+   drm_gem_shmem_resv_assert_held(shmem);
 
kref_put(>pages_pin_count, drm_gem_shmem_kref_unpin_pages);
 }
@@ -357,7 +370,7 @@ int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object 
*shmem,
} else {
pgprot_t prot = PAGE_KERNEL;
 
-   dma_resv_assert_held(shmem->base.resv);
+   drm_gem_shmem_resv_assert_held(shmem);
 
if (kref_get_unless_zero(>vmap_use_count)) {
iosys_map_set_vaddr(map, shmem->vaddr);
@@ -426,7 +439,7 @@ void drm_gem_shmem_vunmap_locked(struct 
drm_gem_shmem_object *shmem,
if (obj->import_attach) {
dma_buf_vunmap(obj->import_attach->dmabuf, map);
} else {
-   dma_resv_assert_held(shmem->base.resv);
+   drm_gem_shmem_resv_assert_held(shmem);
kref_put(>vmap_use_count, drm_gem_shmem_kref_vunmap);
}
 
@@ -462,7 +475,7 @@ drm_gem_shmem_create_with_handle(struct drm_file *file_priv,
  */
 int drm_gem_shmem_madvise_locked(struct drm_gem_shmem_object *shmem, int madv)
 {
-   dma_resv_assert_held(shmem->base.resv);
+   drm_gem_shmem_resv_assert_held(shmem);
 
if (shmem->madv >= 0)
shmem->madv = madv;
@@ -478,7 +491,7 @@ void