[PATCH 1/2] drm: make idr_mutex a spinlock

[David Herrmann]

Hi

On Fri, Aug 29, 2014 at 2:53 PM, Daniel Vetter <daniel at ffwll.ch> 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 at gmail.com>
>
> 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