On 07/20, Paul E. McKenney wrote:
>
> On Wed, Jul 20, 2016 at 07:16:03PM +0200, Oleg Nesterov wrote:
>
> > Now, suppose we add the additional enter/exit's:
> >
> > freeze_super(sb)
> > {
> > // this doesn't block
> > __rcu_sync_enter(SEM3);
> > __rcu_sync_enter(SEM2);
> > __rcu_sync_enter(SEM1);
> >
> > down_write(&sb->s_umount);
> > if (NEED_TO_FREEZE) {
> > percpu_down_write(SEM1);
>
> The above waits for the grace period initiated by __rcu_sync_enter(),
> correct? Presumably "yes", because it will invoke rcu_sync_enter(), which
> will see the state as GP_ENTER, and will thus wait.
But if down_write() blocks and/or NEED_TO_FREEZE takes some time it
could already see the GP_PASSED state, or at least it can sleep less.
> But your point is that if !NEED_TO_FREEZE, we will get here without
> waiting for a grace period.
>
> But why aren't the __rcu_sync_enter() and rcu_sync_exit() calls inside
> the "if" statement?
Yes, if we do __rcu_sync_enter() inside "if", then rcu_sync_exit() can't
hit GP_ENTER.
But why we should disallow this use-case? It does not complicate the code
at all.
And see above, we want to initiate the GP "asap", so that we will sleep
less later. Although yes, freeze_super() is not the best example. And
__cgroup_procs_write() too, but note that cgroup_kn_lock_live() is rather
heavy, takes the global locks, and can fail. So (ignoring the fact we
are going to switch cgroup_threadgroup_rwsem into the slow mode for now)
__rcu_sync_enter() at the start could help to lessen the time
percpu_down_write(cgroup_threadgroup_rwsem) sleeps with the cgroup_mutex
held.
> That aside, would it make sense to name __rcu_sync_enter() something
> like rcu_sync_begin_to_enter(), rcu_sync_pre_enter() or some such?
> Something to make it clear that it just starts the job and that something
> else is needed to finish it.
Sure. Agreed, will rename.
> And here is an updated state table. I do not yet separately call out
> __rcu_sync_enter(), though without it the rcu_sync_exit() transition
> out of state B cannot happen.
Thanks! I'll try to double-check it.
Oleg.
