Messages in this thread | | | Date | Wed, 30 Jul 2014 11:24:41 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu 8/9] rcu: Make RCU-tasks track exiting tasks |
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On Wed, Jul 30, 2014 at 07:04:42PM +0200, Oleg Nesterov wrote: > On 07/28, Paul E. McKenney wrote: > > > > This commit adds synchronization with exiting tasks, so that RCU-tasks > > avoids waiting on tasks that no longer exist. > > I don't understand this patch yet, but it seems that it adds more than > just synchronization with exiting tasks?
There was also a bit of code reorganization to keep indentation level down to a dull roar.
> > + ACCESS_ONCE(t->rcu_tasks_holdout) = 1; > > + spin_unlock(&t->rcu_tasks_lock); > > + smp_mb(); /* Order ->rcu_tasks_holdout store before "if". */ > > + if (t == current || !ACCESS_ONCE(t->on_rq) || is_idle_task(t)) { > > + smp_store_release(&t->rcu_tasks_holdout, 0); > > + goto next_thread; > > + } > > This should avoid the race with schedule()->rcu_note_voluntary_context_switch(), > right? > > > - rcu_read_lock(); > > - do_each_thread(g, t) { > > - if (t != current && ACCESS_ONCE(t->on_rq) && > > - !is_idle_task(t)) { > > - t->rcu_tasks_holdout = 1; > > Because before this patch the code looks obviously racy, a task can do > sleep(FOREVER) and block rcu_tasks_kthread() if it reads ->on_rq == 1 > after rcu_note_voluntary_context_switch() was already called. > > However, I am not sure this race is actually closed even after this > change... why rcu_note_voluntary_context_switch() can not miss > ->rcu_tasks_holdout != 0 ?
Good point, I need to add a !ACCESS_ONCE(t->on_rq) when scanning the list of tasks blocking the grace period. I also need to handle NO_HZ_FULL, but that comes later.
Thanx, Paul
> OK, it seems that you are going to send the next version anyway, so > please ignore. > > Oleg. >
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