On Sun, 19 Nov 2006, Oleg Nesterov wrote:
> > What happens if synchronize_xxx manages to execute inbetween
> > xxx_read_lock's
> >
> > idx = sp->completed & 0x1;
> > atomic_inc(sp->ctr + idx);
> >
> > statements?
>
> Oops. I forgot about explicit mb() before sp->completed++ in synchronize_xxx().
>
> So synchronize_xxx() should do
>
> smp_mb();
> idx = sp->completed++ & 0x1;
>
> for (;;) { ... }
>
> > You see, there's no way around using synchronize_sched().
>
> With this change I think we are safe.
>
> If synchronize_xxx() increments ->completed in between, the caller of
> xxx_read_lock() will see all memory ops (started before synchronize_xxx())
> completed. It is ok that synchronize_xxx() returns immediately.
Yes, the reader will see a consistent picture, but it will have
incremented the wrong element of sp->ctr[]. What happens if another
synchronize_xxx() occurs while the reader is still running?
Alan
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