Messages in this thread | | | Date | Mon, 17 Mar 2008 23:17:41 +0300 | From | Oleg Nesterov <> | Subject | Re: [PATCH] fix misplaced mb() in rcu_enter/exit_nohz() |
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(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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