Hi On Fri, Aug 29, 2014 at 2:53 PM, Daniel Vetter <daniel@xxxxxxxx> wrote: > On Fri, Aug 29, 2014 at 02:01:00PM +0200, David Herrmann wrote: >> There is no reason to use a heavy mutex for idr protection. Use a spinlock >> and make idr-allocation use idr_preload(). >> >> This patch also makes mode-object lookup irq-save, in case you ever wanna >> lookup modeset objects from interrupts. This is just a side-effect of >> avoiding a mutex. >> >> Signed-off-by: David Herrmann <dh.herrmann@xxxxxxxxx> > > I've thought irqsave/restore are terribly serializing instructions, so > this might actually be slower than a plain mutex. And imo if it doesn't > show up in profiles it's not worth to optimize it - generally mutexes are > really fast and in most cases already nicely degenerate to spinlocks > anyway. Honestly, this patch is less about speed than 'correctness'. Sure, a mutex is just a spin-lock in low-contention cases and there really is no high-contention here. However, spin-locks are the major lock-type for pure data. mutexes only make sense when you have to lock data structures _while_ performing operations that can sleep. Using a spin-lock here prevents people from doing stupid things while holding this lock. And really, this lock is about ID registration and deregistration, nothing else. Btw., I can happily turn all those lock/unlock sequences into spin_lock() and spin_unlock() so we ignore irq-contexts completely, if that's the only issue. Thanks David _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
