On 11/19, Paul E. McKenney wrote:
>
> On Sun, Nov 19, 2006 at 11:55:16PM +0300, Oleg Nesterov wrote:
> > 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.
>
> Let me take Alan's example one step further:
>
> o CPU 0 starts executing xxx_read_lock(), but is interrupted
> (or whatever) just before the atomic_inc().
>
> o CPU 1 executes synchronize_xxx() to completion, which it
> can because CPU 0 has not yet incremented the counter.
Let's suppose for simplicity that CPU 1 does "classical"
old = global_ptr;
global_ptr = new_value();
before synchronize_xxx(), and ->completed == 0.
Now, synchronize_xxx() sets ->completed == 1. Because of mb()
'global_ptr = new_value()' is completed.
> o CPU 0 returns from interrupt and completes xxx_read_lock(),
> but has incremented the wrong counter.
->completed == 1, it is not so wrong, see below
> 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).
it sees the new value in global_ptr, we are safe.
> o CPU 1 executes another synchronize_xxx(). This completes
> immediately because CPU 1 has the wrong counter incremented.
No, it will notice .ctr[1] != 1 and wait.
> 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.
CPU 1 continues only when CPU 0 does read_unlock(/*completed*/ 1),
we are safe.
Safe?
Oleg.
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