Subject: Re: [PATCH v6 04/46] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks

From: Oleg Nesterov <[email-masked]>
Date: Thu, 28 Feb 2013 19:00:07 +0100

On 02/28, Michel Lespinasse wrote:
>
> On Thu, Feb 28, 2013 at 3:25 AM, Oleg Nesterov <[email-masked]> wrote:
> > On 02/27, Michel Lespinasse wrote:
> >>
> >> +void lg_rwlock_local_read_lock(struct lgrwlock *lgrw)
> >> +{
> >> +       preempt_disable();
> >> +
> >> +       if (__this_cpu_read(*lgrw->local_refcnt) ||
> >> +           arch_spin_trylock(this_cpu_ptr(lgrw->lglock->lock))) {
> >> +               __this_cpu_inc(*lgrw->local_refcnt);
> >
> > Please look at __this_cpu_generic_to_op(). You need this_cpu_inc()
> > to avoid the race with irs. The same for _read_unlock.
>
> Hmmm, I was thinking that this was safe because while interrupts might
> modify local_refcnt to acquire a nested read lock, they are expected
> to release that lock as well which would set local_refcnt back to its
> original value ???

Yes, yes, this is correct.

I meant that (in general, x86 is fine) __this_cpu_inc() itself is not
irq-safe. It simply does "pcp += 1".

this_cpu_inc() is fine, _this_cpu_generic_to_op() does cli/sti around.

I know this only because I did the same mistake recently, and Srivatsa
explained the problem to me ;)

Oleg.