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@xxxxxxxxxxxxxx>
> > > ---
> > > 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.