Re: [RFC][PATCH] mutex: Optimize mutex_trylock() fast-path

[Davidlohr Bueso]

On Wed, 01 Jun 2016, Peter Zijlstra wrote:

> A while back Viro posted a number of 'interesting' mutex_is_locked()
> users on IRC, one of those was RCU.
>
> RCU seems to use mutex_is_locked() to avoid doing mutex_trylock(), the
> regular load before modify pattern.
>
> While the use isn't wrong per se, its curious in that its needed at all,
> mutex_trylock() should be good enough on its own to avoid the pointless
> cacheline bounces.

Yeah, and we use ccas just about everywhere else for mutexes.

> So fix those and remove the mutex_is_locked() (ab)use from RCU.
>
> Reported-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Acked-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>