linux-kernel - Re: [PATCH RFC tip/core/rcu 8/9] rcu: Make RCU-tasks track exiting tasks

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 30 Jul 2014 11:24:41 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, bobby.prani@...il.com
Subject: Re: [PATCH RFC tip/core/rcu 8/9] rcu: Make RCU-tasks track exiting
 tasks

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.
> 

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/