[PATCH v2] dma-buf/fence: Take refcount on the module that owns the fence

[PATCH v2] dma-buf/fence: Take refcount on the module that owns the fence

[Akhil P Oommen]

Each fence object holds function pointers of the module that initialized
it. Allowing the module to unload before this fence's release is
catastrophic. So, keep a refcount on the module until the fence is
released.

Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
---
Changes in v2:
- added description for the new function parameter.

 drivers/dma-buf/dma-fence.c | 16 +++++++++++++---
 include/linux/dma-fence.h   | 10 ++++++++--
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 4edb9fd..2aaa44e 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -18,6 +18,7 @@
  * more details.
  */
 
+#include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/export.h>
 #include <linux/atomic.h>
@@ -168,6 +169,7 @@ void dma_fence_release(struct kref *kref)
 {
 	struct dma_fence *fence =
 		container_of(kref, struct dma_fence, refcount);
+	struct module *module = fence->owner;
 
 	trace_dma_fence_destroy(fence);
 
@@ -178,6 +180,8 @@ void dma_fence_release(struct kref *kref)
 		fence->ops->release(fence);
 	else
 		dma_fence_free(fence);
+
+	module_put(module);
 }
 EXPORT_SYMBOL(dma_fence_release);
 
@@ -541,6 +545,7 @@ struct default_wait_cb {
 
 /**
  * dma_fence_init - Initialize a custom fence.
+ * @module:	[in]	the module that calls this API
  * @fence:	[in]	the fence to initialize
  * @ops:	[in]	the dma_fence_ops for operations on this fence
  * @lock:	[in]	the irqsafe spinlock to use for locking this fence
@@ -556,8 +561,9 @@ struct default_wait_cb {
  * to check which fence is later by simply using dma_fence_later.
  */
 void
-dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
-	       spinlock_t *lock, u64 context, unsigned seqno)
+_dma_fence_init(struct module *module, struct dma_fence *fence,
+		const struct dma_fence_ops *ops, spinlock_t *lock,
+		u64 context, unsigned seqno)
 {
 	BUG_ON(!lock);
 	BUG_ON(!ops || !ops->wait || !ops->enable_signaling ||
@@ -571,7 +577,11 @@ struct default_wait_cb {
 	fence->seqno = seqno;
 	fence->flags = 0UL;
 	fence->error = 0;
+	fence->owner = module;
+
+	if (!try_module_get(module))
+		fence->owner = NULL;
 
 	trace_dma_fence_init(fence);
 }
-EXPORT_SYMBOL(dma_fence_init);
+EXPORT_SYMBOL(_dma_fence_init);
diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
index eb9b05a..8159125 100644
--- a/include/linux/dma-fence.h
+++ b/include/linux/dma-fence.h
@@ -36,6 +36,8 @@
 
 /**
  * struct dma_fence - software synchronization primitive
+ * @owner: the module that contains fence_ops functions.
+ *	   Usually THIS_MODULE.
  * @refcount: refcount for this fence
  * @ops: dma_fence_ops associated with this fence
  * @rcu: used for releasing fence with kfree_rcu
@@ -71,6 +73,7 @@
  * been completed, or never called at all.
  */
 struct dma_fence {
+	struct module *owner;
 	struct kref refcount;
 	const struct dma_fence_ops *ops;
 	struct rcu_head rcu;
@@ -249,8 +252,11 @@ struct dma_fence_ops {
 				   char *str, int size);
 };
 
-void dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
-		    spinlock_t *lock, u64 context, unsigned seqno);
+#define dma_fence_init(fence, ops, lock, context, seqno) _dma_fence_init( \
+		THIS_MODULE, fence, ops, lock, context, seqno)
+void _dma_fence_init(struct module *module, struct dma_fence *fence,
+		const struct dma_fence_ops *ops, spinlock_t *lock, u64 context,
+		unsigned seqno);
 
 void dma_fence_release(struct kref *kref);
 void dma_fence_free(struct dma_fence *fence);
-- 
1.9.1

Re: [PATCH v2] dma-buf/fence: Take refcount on the module that owns the fence

[Gustavo Padovan]

Hi Akhil,

On Fri, 2018-06-22 at 15:10 +0530, Akhil P Oommen wrote:
> Each fence object holds function pointers of the module that
> initialized
> it. Allowing the module to unload before this fence's release is
> catastrophic. So, keep a refcount on the module until the fence is
> released.
> 
> Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
> ---
> Changes in v2:
> - added description for the new function parameter.
> 
>  drivers/dma-buf/dma-fence.c | 16 +++++++++++++---
>  include/linux/dma-fence.h   | 10 ++++++++--
>  2 files changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-
> fence.c
> index 4edb9fd..2aaa44e 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -18,6 +18,7 @@
>   * more details.
>   */
>  
> +#include <linux/module.h>
>  #include <linux/slab.h>
>  #include <linux/export.h>
>  #include <linux/atomic.h>
> @@ -168,6 +169,7 @@ void dma_fence_release(struct kref *kref)
>  {
>  	struct dma_fence *fence =
>  		container_of(kref, struct dma_fence, refcount);
> +	struct module *module = fence->owner;
>  
>  	trace_dma_fence_destroy(fence);
>  
> @@ -178,6 +180,8 @@ void dma_fence_release(struct kref *kref)
>  		fence->ops->release(fence);
>  	else
>  		dma_fence_free(fence);
> +
> +	module_put(module);
>  }
>  EXPORT_SYMBOL(dma_fence_release);
>  
> @@ -541,6 +545,7 @@ struct default_wait_cb {
>  
>  /**
>   * dma_fence_init - Initialize a custom fence.
> + * @module:	[in]	the module that calls this API
>   * @fence:	[in]	the fence to initialize
>   * @ops:	[in]	the dma_fence_ops for operations on this
> fence
>   * @lock:	[in]	the irqsafe spinlock to use for locking
> this fence
> @@ -556,8 +561,9 @@ struct default_wait_cb {
>   * to check which fence is later by simply using dma_fence_later.
>   */
>  void
> -dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops
> *ops,
> -	       spinlock_t *lock, u64 context, unsigned seqno)
> +_dma_fence_init(struct module *module, struct dma_fence *fence,
> +		const struct dma_fence_ops *ops, spinlock_t *lock,
> +		u64 context, unsigned seqno)
>  {
>  	BUG_ON(!lock);
>  	BUG_ON(!ops || !ops->wait || !ops->enable_signaling ||
> @@ -571,7 +577,11 @@ struct default_wait_cb {
>  	fence->seqno = seqno;
>  	fence->flags = 0UL;
>  	fence->error = 0;
> +	fence->owner = module;
> +
> +	if (!try_module_get(module))
> +		fence->owner = NULL;
>  
>  	trace_dma_fence_init(fence);
>  }
> -EXPORT_SYMBOL(dma_fence_init);
> +EXPORT_SYMBOL(_dma_fence_init);

Do we still need to export the symbol, it won't be called from outside
anymore? Other than that looks good to me:

Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>

Gustavo

Re: [PATCH v2] dma-buf/fence: Take refcount on the module that owns the fence

[Chris Wilson]

Quoting Gustavo Padovan (2018-06-22 11:04:16)
> Hi Akhil,
> 
> On Fri, 2018-06-22 at 15:10 +0530, Akhil P Oommen wrote:
> > Each fence object holds function pointers of the module that
> > initialized
> > it. Allowing the module to unload before this fence's release is
> > catastrophic. So, keep a refcount on the module until the fence is
> > released.
> > 
> > Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
> > ---
> > Changes in v2:
> > - added description for the new function parameter.
> > 
> >  drivers/dma-buf/dma-fence.c | 16 +++++++++++++---
> >  include/linux/dma-fence.h   | 10 ++++++++--
> >  2 files changed, 21 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-
> > fence.c
> > index 4edb9fd..2aaa44e 100644
> > --- a/drivers/dma-buf/dma-fence.c
> > +++ b/drivers/dma-buf/dma-fence.c
> > @@ -18,6 +18,7 @@
> >   * more details.
> >   */
> >  
> > +#include <linux/module.h>
> >  #include <linux/slab.h>
> >  #include <linux/export.h>
> >  #include <linux/atomic.h>
> > @@ -168,6 +169,7 @@ void dma_fence_release(struct kref *kref)
> >  {
> >       struct dma_fence *fence =
> >               container_of(kref, struct dma_fence, refcount);
> > +     struct module *module = fence->owner;
> >  
> >       trace_dma_fence_destroy(fence);
> >  
> > @@ -178,6 +180,8 @@ void dma_fence_release(struct kref *kref)
> >               fence->ops->release(fence);
> >       else
> >               dma_fence_free(fence);
> > +
> > +     module_put(module);
> >  }
> >  EXPORT_SYMBOL(dma_fence_release);
> >  
> > @@ -541,6 +545,7 @@ struct default_wait_cb {
> >  
> >  /**
> >   * dma_fence_init - Initialize a custom fence.
> > + * @module:  [in]    the module that calls this API
> >   * @fence:   [in]    the fence to initialize
> >   * @ops:     [in]    the dma_fence_ops for operations on this
> > fence
> >   * @lock:    [in]    the irqsafe spinlock to use for locking
> > this fence
> > @@ -556,8 +561,9 @@ struct default_wait_cb {
> >   * to check which fence is later by simply using dma_fence_later.
> >   */
> >  void
> > -dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops
> > *ops,
> > -            spinlock_t *lock, u64 context, unsigned seqno)
> > +_dma_fence_init(struct module *module, struct dma_fence *fence,
> > +             const struct dma_fence_ops *ops, spinlock_t *lock,
> > +             u64 context, unsigned seqno)
> >  {
> >       BUG_ON(!lock);
> >       BUG_ON(!ops || !ops->wait || !ops->enable_signaling ||
> > @@ -571,7 +577,11 @@ struct default_wait_cb {
> >       fence->seqno = seqno;
> >       fence->flags = 0UL;
> >       fence->error = 0;
> > +     fence->owner = module;
> > +
> > +     if (!try_module_get(module))
> > +             fence->owner = NULL;
> >  
> >       trace_dma_fence_init(fence);
> >  }
> > -EXPORT_SYMBOL(dma_fence_init);
> > +EXPORT_SYMBOL(_dma_fence_init);
> 
> Do we still need to export the symbol, it won't be called from outside
> anymore? Other than that looks good to me:

There's a big drawback in that a module reference is often insufficient,
and that a reference on the driver (or whatever is required for the
lifetime of the fence) will already hold the module reference.

Considering that we want a few 100k fences in flight per second, is
there no other way to only export a fence with a module reference?
-Chris

Re: [PATCH v2] dma-buf/fence: Take refcount on the module that owns the fence

[Akhil P Oommen]

On 6/22/2018 3:38 PM, Chris Wilson wrote:
> Quoting Gustavo Padovan (2018-06-22 11:04:16)
>> Hi Akhil,
>>
>> On Fri, 2018-06-22 at 15:10 +0530, Akhil P Oommen wrote:
>>> Each fence object holds function pointers of the module that
>>> initialized
>>> it. Allowing the module to unload before this fence's release is
>>> catastrophic. So, keep a refcount on the module until the fence is
>>> released.
>>>
>>> Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
>>> ---
>>> Changes in v2:
>>> - added description for the new function parameter.
>>>
>>>   drivers/dma-buf/dma-fence.c | 16 +++++++++++++---
>>>   include/linux/dma-fence.h   | 10 ++++++++--
>>>   2 files changed, 21 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-
>>> fence.c
>>> index 4edb9fd..2aaa44e 100644
>>> --- a/drivers/dma-buf/dma-fence.c
>>> +++ b/drivers/dma-buf/dma-fence.c
>>> @@ -18,6 +18,7 @@
>>>    * more details.
>>>    */
>>>   
>>> +#include <linux/module.h>
>>>   #include <linux/slab.h>
>>>   #include <linux/export.h>
>>>   #include <linux/atomic.h>
>>> @@ -168,6 +169,7 @@ void dma_fence_release(struct kref *kref)
>>>   {
>>>        struct dma_fence *fence =
>>>                container_of(kref, struct dma_fence, refcount);
>>> +     struct module *module = fence->owner;
>>>   
>>>        trace_dma_fence_destroy(fence);
>>>   
>>> @@ -178,6 +180,8 @@ void dma_fence_release(struct kref *kref)
>>>                fence->ops->release(fence);
>>>        else
>>>                dma_fence_free(fence);
>>> +
>>> +     module_put(module);
>>>   }
>>>   EXPORT_SYMBOL(dma_fence_release);
>>>   
>>> @@ -541,6 +545,7 @@ struct default_wait_cb {
>>>   
>>>   /**
>>>    * dma_fence_init - Initialize a custom fence.
>>> + * @module:  [in]    the module that calls this API
>>>    * @fence:   [in]    the fence to initialize
>>>    * @ops:     [in]    the dma_fence_ops for operations on this
>>> fence
>>>    * @lock:    [in]    the irqsafe spinlock to use for locking
>>> this fence
>>> @@ -556,8 +561,9 @@ struct default_wait_cb {
>>>    * to check which fence is later by simply using dma_fence_later.
>>>    */
>>>   void
>>> -dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops
>>> *ops,
>>> -            spinlock_t *lock, u64 context, unsigned seqno)
>>> +_dma_fence_init(struct module *module, struct dma_fence *fence,
>>> +             const struct dma_fence_ops *ops, spinlock_t *lock,
>>> +             u64 context, unsigned seqno)
>>>   {
>>>        BUG_ON(!lock);
>>>        BUG_ON(!ops || !ops->wait || !ops->enable_signaling ||
>>> @@ -571,7 +577,11 @@ struct default_wait_cb {
>>>        fence->seqno = seqno;
>>>        fence->flags = 0UL;
>>>        fence->error = 0;
>>> +     fence->owner = module;
>>> +
>>> +     if (!try_module_get(module))
>>> +             fence->owner = NULL;
>>>   
>>>        trace_dma_fence_init(fence);
>>>   }
>>> -EXPORT_SYMBOL(dma_fence_init);
>>> +EXPORT_SYMBOL(_dma_fence_init);
>> Do we still need to export the symbol, it won't be called from outside
>> anymore? Other than that looks good to me:
Yes. Because dma_fence_init() is now a macro that resolves to 
_dma_fence_init().
> There's a big drawback in that a module reference is often insufficient,
> and that a reference on the driver (or whatever is required for the
> lifetime of the fence) will already hold the module reference.
I didn't quite understand what you meant here. Could you please elaborate?
>
> Considering that we want a few 100k fences in flight per second, is
> there no other way to only export a fence with a module reference?
> -Chris

Thanks,
Akhil.

Re: [PATCH v2] dma-buf/fence: Take refcount on the module that owns the fence

[Daniel Vetter]

On Fri, Jun 22, 2018 at 11:08:48AM +0100, Chris Wilson wrote:
> Quoting Gustavo Padovan (2018-06-22 11:04:16)
> > Hi Akhil,
> > 
> > On Fri, 2018-06-22 at 15:10 +0530, Akhil P Oommen wrote:
> > > Each fence object holds function pointers of the module that
> > > initialized
> > > it. Allowing the module to unload before this fence's release is
> > > catastrophic. So, keep a refcount on the module until the fence is
> > > released.
> > > 
> > > Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
> > > ---
> > > Changes in v2:
> > > - added description for the new function parameter.
> > > 
> > >  drivers/dma-buf/dma-fence.c | 16 +++++++++++++---
> > >  include/linux/dma-fence.h   | 10 ++++++++--
> > >  2 files changed, 21 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-
> > > fence.c
> > > index 4edb9fd..2aaa44e 100644
> > > --- a/drivers/dma-buf/dma-fence.c
> > > +++ b/drivers/dma-buf/dma-fence.c
> > > @@ -18,6 +18,7 @@
> > >   * more details.
> > >   */
> > >  
> > > +#include <linux/module.h>
> > >  #include <linux/slab.h>
> > >  #include <linux/export.h>
> > >  #include <linux/atomic.h>
> > > @@ -168,6 +169,7 @@ void dma_fence_release(struct kref *kref)
> > >  {
> > >       struct dma_fence *fence =
> > >               container_of(kref, struct dma_fence, refcount);
> > > +     struct module *module = fence->owner;
> > >  
> > >       trace_dma_fence_destroy(fence);
> > >  
> > > @@ -178,6 +180,8 @@ void dma_fence_release(struct kref *kref)
> > >               fence->ops->release(fence);
> > >       else
> > >               dma_fence_free(fence);
> > > +
> > > +     module_put(module);
> > >  }
> > >  EXPORT_SYMBOL(dma_fence_release);
> > >  
> > > @@ -541,6 +545,7 @@ struct default_wait_cb {
> > >  
> > >  /**
> > >   * dma_fence_init - Initialize a custom fence.
> > > + * @module:  [in]    the module that calls this API
> > >   * @fence:   [in]    the fence to initialize
> > >   * @ops:     [in]    the dma_fence_ops for operations on this
> > > fence
> > >   * @lock:    [in]    the irqsafe spinlock to use for locking
> > > this fence
> > > @@ -556,8 +561,9 @@ struct default_wait_cb {
> > >   * to check which fence is later by simply using dma_fence_later.
> > >   */
> > >  void
> > > -dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops
> > > *ops,
> > > -            spinlock_t *lock, u64 context, unsigned seqno)
> > > +_dma_fence_init(struct module *module, struct dma_fence *fence,
> > > +             const struct dma_fence_ops *ops, spinlock_t *lock,
> > > +             u64 context, unsigned seqno)
> > >  {
> > >       BUG_ON(!lock);
> > >       BUG_ON(!ops || !ops->wait || !ops->enable_signaling ||
> > > @@ -571,7 +577,11 @@ struct default_wait_cb {
> > >       fence->seqno = seqno;
> > >       fence->flags = 0UL;
> > >       fence->error = 0;
> > > +     fence->owner = module;
> > > +
> > > +     if (!try_module_get(module))
> > > +             fence->owner = NULL;
> > >  
> > >       trace_dma_fence_init(fence);
> > >  }
> > > -EXPORT_SYMBOL(dma_fence_init);
> > > +EXPORT_SYMBOL(_dma_fence_init);
> > 
> > Do we still need to export the symbol, it won't be called from outside
> > anymore? Other than that looks good to me:
> 
> There's a big drawback in that a module reference is often insufficient,
> and that a reference on the driver (or whatever is required for the
> lifetime of the fence) will already hold the module reference.
> 
> Considering that we want a few 100k fences in flight per second, is
> there no other way to only export a fence with a module reference?

We'd need to make the timeline a full-blown object (Maarten owes me one
for that design screw-up), and then we could stuff all these things in
there.

And I think that's the right fix, since try_module_get for every
dma_fence_init just ain't cool really :-)
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [PATCH v2] dma-buf/fence: Take refcount on the module that owns the fence

[Daniel Vetter]

On Mon, Jun 25, 2018 at 09:21:15PM +0530, Akhil P Oommen wrote:
> 
> 
> On 6/25/2018 1:20 PM, Daniel Vetter wrote:
> > On Fri, Jun 22, 2018 at 11:08:48AM +0100, Chris Wilson wrote:
> > > Quoting Gustavo Padovan (2018-06-22 11:04:16)
> > > > Hi Akhil,
> > > > 
> > > > On Fri, 2018-06-22 at 15:10 +0530, Akhil P Oommen wrote:
> > > > > Each fence object holds function pointers of the module that
> > > > > initialized
> > > > > it. Allowing the module to unload before this fence's release is
> > > > > catastrophic. So, keep a refcount on the module until the fence is
> > > > > released.
> > > > > 
> > > > > Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
> > > > > ---
> > > > > Changes in v2:
> > > > > - added description for the new function parameter.
> > > > > 
> > > > >   drivers/dma-buf/dma-fence.c | 16 +++++++++++++---
> > > > >   include/linux/dma-fence.h   | 10 ++++++++--
> > > > >   2 files changed, 21 insertions(+), 5 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-
> > > > > fence.c
> > > > > index 4edb9fd..2aaa44e 100644
> > > > > --- a/drivers/dma-buf/dma-fence.c
> > > > > +++ b/drivers/dma-buf/dma-fence.c
> > > > > @@ -18,6 +18,7 @@
> > > > >    * more details.
> > > > >    */
> > > > > +#include <linux/module.h>
> > > > >   #include <linux/slab.h>
> > > > >   #include <linux/export.h>
> > > > >   #include <linux/atomic.h>
> > > > > @@ -168,6 +169,7 @@ void dma_fence_release(struct kref *kref)
> > > > >   {
> > > > >        struct dma_fence *fence =
> > > > >                container_of(kref, struct dma_fence, refcount);
> > > > > +     struct module *module = fence->owner;
> > > > >        trace_dma_fence_destroy(fence);
> > > > > @@ -178,6 +180,8 @@ void dma_fence_release(struct kref *kref)
> > > > >                fence->ops->release(fence);
> > > > >        else
> > > > >                dma_fence_free(fence);
> > > > > +
> > > > > +     module_put(module);
> > > > >   }
> > > > >   EXPORT_SYMBOL(dma_fence_release);
> > > > > @@ -541,6 +545,7 @@ struct default_wait_cb {
> > > > >   /**
> > > > >    * dma_fence_init - Initialize a custom fence.
> > > > > + * @module:  [in]    the module that calls this API
> > > > >    * @fence:   [in]    the fence to initialize
> > > > >    * @ops:     [in]    the dma_fence_ops for operations on this
> > > > > fence
> > > > >    * @lock:    [in]    the irqsafe spinlock to use for locking
> > > > > this fence
> > > > > @@ -556,8 +561,9 @@ struct default_wait_cb {
> > > > >    * to check which fence is later by simply using dma_fence_later.
> > > > >    */
> > > > >   void
> > > > > -dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops
> > > > > *ops,
> > > > > -            spinlock_t *lock, u64 context, unsigned seqno)
> > > > > +_dma_fence_init(struct module *module, struct dma_fence *fence,
> > > > > +             const struct dma_fence_ops *ops, spinlock_t *lock,
> > > > > +             u64 context, unsigned seqno)
> > > > >   {
> > > > >        BUG_ON(!lock);
> > > > >        BUG_ON(!ops || !ops->wait || !ops->enable_signaling ||
> > > > > @@ -571,7 +577,11 @@ struct default_wait_cb {
> > > > >        fence->seqno = seqno;
> > > > >        fence->flags = 0UL;
> > > > >        fence->error = 0;
> > > > > +     fence->owner = module;
> > > > > +
> > > > > +     if (!try_module_get(module))
> > > > > +             fence->owner = NULL;
> > > > >        trace_dma_fence_init(fence);
> > > > >   }
> > > > > -EXPORT_SYMBOL(dma_fence_init);
> > > > > +EXPORT_SYMBOL(_dma_fence_init);
> > > > Do we still need to export the symbol, it won't be called from outside
> > > > anymore? Other than that looks good to me:
> > > There's a big drawback in that a module reference is often insufficient,
> > > and that a reference on the driver (or whatever is required for the
> > > lifetime of the fence) will already hold the module reference.
> > > 
> > > Considering that we want a few 100k fences in flight per second, is
> > > there no other way to only export a fence with a module reference?
> > We'd need to make the timeline a full-blown object (Maarten owes me one
> > for that design screw-up), and then we could stuff all these things in
> > there.
> > 
> > And I think that's the right fix, since try_module_get for every
> > dma_fence_init just ain't cool really :-)
> > -Daniel
> Thanks for the feedback, Daniel.
> I see your point, but I am not sure how much impact an extra refcounting
> would create considering the whole effort of setting up a new fence. Also,
> this refcounting is not required for built-in modules.
> 
> As of now, unloading a kernel module that uses fence_init() is an easy way
> to bring down the system. This patch simply fixes that. What you have
> suggested sounds like a non-trivial effort which someone who is more
> familiar with this code base can do a better job than me. Perhaps we can
> take this patch now to fix the issue at hand and later somebody else can
> share a more optimal solution. :)

Module unload is a developer-only feature for a reason. Given that I don't
think fixing this with a hack is the right approach. And dma_fence_init()
is supposed to be really fast.

Also note that you can fix this already for your own driver by simply
waiting for all pending dma_fences to get released, so I don't think it's
super-important to land this asap.

Yes the real fix is a bit more involved, but shouldn't be too hard to pull
off really.
-Daniel

> 
> @Gustavo & @Sumit, I would like the maintainers to take a decision here.
> 
> -Akhil.

> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch