On 11/19, Paul E. McKenney wrote:
>
> On Mon, Nov 20, 2006 at 12:17:31AM +0300, Oleg Nesterov wrote:
> >
> > It will wait for xxx_read_unlock() on reader's side. And for this reason
> > this idx in fact is not exactly wrong :)
>
> I am not seeing this.
>
> Let's assume sp->completed starts out zero.
>
> o CPU 0 starts executing xxx_read_lock(), but is interrupted
> (or whatever) just before the atomic_inc(). Upon return,
> it will increment sp->ctr[0].
Right.
> o CPU 1 executes synchronize_xxx() to completion, which it
> can because CPU 0 has not yet incremented the counter.
> It waited on sp->ctr[0], and incremented sp->completed to 1.
>
> o CPU 0 returns from interrupt and completes xxx_read_lock(),
> but has incremented sp->ctr[0].
>
> o CPU 0 continues into its critical section, picking up a
> pointer to an xxx-protected data structure (or, in Jens's
> case starting an xxx-protected I/O).
>
> o CPU 1 executes another synchronize_xxx(). This completes
> immediately because it is waiting for sp->ctr[1] to go
> to zero, but CPU 0 incremented sp->ctr[0]. (Right?)
Right!
> o CPU 1 continues, either freeing a data structure while
> CPU 0 is still referencing it, or, in Jens's case, completing
> an I/O barrier while there is still outstanding I/O.
>
> Or am I missing something?
No, it is me.
Alan, Paul, thanks a lot for your patience!
Oleg.
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