Re: [patch 1/3] mutex subsystem: trylock

[Ingo Molnar <mingo@elte.hu>]

* Arjan van de Ven <arjan@infradead.org> wrote:

> > + * 1) if the exclusive store fails we fail, and
> > + *
> > + * 2) if the decremented value is not zero we don't even attempt the store.
> 
> 
> btw I really think that 1) is wrong. trylock should do everything it 
> can to get the semaphore short of sleeping. Just because some 
> cacheline got written to (which might even be shared!) in the middle 
> of the atomic op is not a good enough reason to fail the trylock imho. 
> Going into the slowpath.. fine. But here it's a quality of 
> implementation issue; you COULD get the semaphore without sleeping (at 
> least probably, you'd have to retry to know for sure) but because 
> something wrote to the same cacheline as the lock... no. that's just 
> not good enough.. sorry.

point. I solved this in my tree by calling the generic trylock <fn> if 
there's an __ex_flag failure in the ARMv6 case. Should be rare (and thus 
the call is under unlikely()), and should thus still enable the fast 
implementation.

	Ingo