On Wed, Nov 26, 2014 at 03:21:30PM +0000, John Harrison wrote: > On 26/11/2014 12:49, Daniel Vetter wrote: > >On Wed, Nov 26, 2014 at 1:12 PM, John Harrison > ><John.C.Harrison@xxxxxxxxx> wrote: > >>The unref_irq is needed for the flip_queued_req as that is still > >>dereferenced from interrupt time. Possibly this one could now be downgraded > >>to a mutex lock but before the recent rebase, the mmio_flip request was also > >>being dereferenced at interrupt time so definitely needed the irq friendly > >>version. Is there a worry that waiting for the mutex lock could stall the > >>flip handler so long that it misses the next flip? > >Yup this is only possible very recently. I've simply dropped the _irq > >and wrapped with struct_mutex locking, but a lockless unref (which > >only grabs the lock on the final kput in the free function) like Chris > >suggested would indeed be tidier. > >-Daniel > > A lockless unref only works if you can guarantee never to call it when you > are holding the last reference. And that is something that is extremely > difficult to prove in advance. Just because the flip code usually releases > its reference before the request has been retired does not mean that it > always will. I have definitely seen it occur the other way around in my > testing. > > Having a 'lockless' unref that can acquire the mutex lock internally is not > much use when you are needing to call it from code that cannot lock (e.g. an > interrupt handler). In that context, the free must be deferred until a later > time. The idea is to implement something like drm_gem_object_unreference_unlocked. It would be mostly for code tideness and self-documentation really. That it avoids a needless mutex grabbing cycle is just a free bonus. And yeah still forbidden to call this from irq contexts or anything else nasty. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
