linux-kernel - Re: while_each_thread() under rcu_read

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Date:	Sat, 19 Jun 2010 08:44:49 -0700
From:	Mandeep Baines <msb@...gle.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Don Zickus <dzickus@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Jerome Marchand <jmarchan@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: while_each_thread() under rcu_read_lock() is broken?

On Fri, Jun 18, 2010 at 10:35 PM, Frederic Weisbecker
<fweisbec@...il.com> wrote:
> On Fri, Jun 18, 2010 at 10:00:54PM -0700, Mandeep Baines wrote:
>> On Fri, Jun 18, 2010 at 12:34 PM, Oleg Nesterov <oleg@...hat.com> wrote:
>> > (add cc's)
>> >
>> > Hmm. Once I sent this patch, I suddenly realized with horror that
>> > while_each_thread() is NOT safe under rcu_read_lock(). Both
>> > do_each_thread/while_each_thread or do/while_each_thread() can
>> > race with exec().
>> >
>> > Yes, it is safe to do next_thread() or next_task(). But:
>> >
>> >        #define while_each_thread(g, t) \
>> >                while ((t = next_thread(t)) != g)
>> >
>> > suppose that t is not the group leader, and it does de_thread() and then
>> > release_task(g). After that next_thread(t) returns t, not g, and the loop
>> > will never stop.
>> >
>> > I _really_ hope I missed something, will recheck tomorrow with the fresh
>> > head. Still I'd like to share my concerns...
>> >
>>
>> Yep. You're right. Not sure what I was thinking. This is only case
>> where do_each_thread
>> is protected by an rcu_read_lock. All others, correctly use read_lock.
>
>
>
> cgroup does too.
> taskstats also uses rcu with while_each_threads, and may be some
> others.
>
> It's not your fault, theses iterators are supposed to be rcu safe,
> we are just encountering a bad race :)
>

Thanks:) Feel less dumb now. My quick grep only turned up hung_task:

$ find . -name \*.c | xargs fgrep -B 10 do_each_thread | grep rcu
./kernel/hung_task.c-   rcu_read_lock();

>
>
>> > If I am right, probably we can fix this, something like
>> >
>> >        #define while_each_thread(g, t) \
>> >                while ((t = next_thread(t)) != g && pid_alive(g))
>> >
>>
>> This seems like a reasonable approach. Maybe call it:
>>
>> while_each_thread_maybe_rcu() :)
>
>
>
> Hmm, no while_each_thread must really be rcu_safe.
>

I didn't realize there were other cases which need while_each_thread to
be rcu-safe. For hung_task, its OK to break out on a release_task(g).
We'll just check the threads we missed on the next iteration.

>
>
>>
>> This makes hung_task a little less useful for failure fencing since
>> this (and rcu_lock_break)
>> increases the potential for never examining all threads but its still
>> a nice lightweight diagnostic
>> for finding bugs.
>
>
>
> In fact may be we could just drop the rcu break, people who really
> care about latencies can use the preemptable version.
>

For large systems, you'd pin a CPU for a very long time checking for
hung_tasks. You'd cause a lot of memory pressure by delaying the
grace period for such a long time. You'd also cause softlockups with
the huge burst of call_rcus being processed by rcu_process_callbacks.

> I know the worry is more about delaying too much the grace period if
> we walk a too long task list, but I don't think it's really a problem.
>
>
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