lkml.org 
[lkml]   [2010]   [Jun]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    SubjectRe: while_each_thread() under rcu_read_lock() is broken?
    From
    On Fri, Jun 18, 2010 at 10:35 PM, Frederic Weisbecker
    <fweisbec@gmail.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@redhat.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.
    >
    >
    --
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2010-06-19 17:47    [W:0.029 / U:297.288 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site