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    SubjectRe: [PATCH] fix misplaced mb() in rcu_enter/exit_nohz()
    (to clarify: my question is completely offtopic to this patch)

    On 03/17, Paul E. McKenney wrote:
    >
    > On Mon, Mar 17, 2008 at 09:30:47PM +0300, Oleg Nesterov wrote:
    > > On 03/16, Paul E. McKenney wrote:
    > > >
    > > > In the process of writing up the mechanical proof of correctness for the
    > > > dynticks/preemptable-RCU interface, I noticed misplaced memory barriers
    > > > in rcu_enter_nohz() and rcu_exit_nohz().
    > >
    > > Can't comment this patch, there is no rcu_enter_nohz() in my rcupreempt.h ;)
    >
    > It is in 2.6.25-rc4 and later. ;-)

    Ah, for some reasons I'm still with -rc2 ...

    > > I'm not sure the code below is up to date, but what I have in
    > > arch/s390/kernel/time.c is:
    > >
    > > stop_hz_timer:
    > >
    > > cpu_set(cpu, nohz_cpu_mask);
    > >
    > > if (rcu_needs_cpu(cpu) || local_softirq_pending()) {
    > > cpu_clear(cpu, nohz_cpu_mask);
    > > return;
    > > }
    > >
    > > Don't we need smp_mb() after cpu_set() ?
    >
    > S390's memory model is quite strong, so it might not be needed.

    OK, in that case we shouldn't worry.

    > In any
    > case, if needed, it goes -before- the cpu_set(), because the problems
    > would arise if prior RCU read-side critical sections were to be reordered
    > to follow this cpu_set(), right?

    No, but it is very possible I missed something.

    What if rcu_needs_cpu(cpu) is executed before cpu_set(cpu, nohz_cpu_mask)?
    It can miss rcu_start_batch() -> rcp->cur++ and return false, but at the
    same time rcu_start_batch() may see nohz_cpu_mask without this CPU.

    No?

    Oleg.



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