On Mon, Nov 20, 2006 at 01:28:47AM +0300, Oleg Nesterov wrote:
> 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!
No problem -- now we just need to work out if Jens's optimization is
safe, either in his situation or in general. And I need to chase down
any remaining bugs in the patch...
Thanx, Paul
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