3.14-rc2 XFS backtrace because irqs_disabled.

[Al Viro]

On Sat, Feb 15, 2014 at 03:22:51PM +0000, Al Viro wrote:
> On Sat, Feb 15, 2014 at 03:27:00PM +0100, Oleg Nesterov wrote:
> 
> > 1. info->q can be already freed if SIGQUEUE_PREALLOC.
> > 
> >    Once get_signal_to_deliver() or any other caller drops ->siglock
> >    another thread can do sys_timer_delete()->sigqueue_free().
> 
> How the devil would it find the sucker?  It's off the list already.

Ouch...  I think I see what you mean.  Let me see if I got it right:
timer->sigq is *not* freed by collect_signal(); it's done by
release_posix_timer() instead, under siglock.  Frankly, this
        /*
         * If it is queued it will be freed when dequeued,
         * like the "regular" sigqueue.
         */
        if (!list_empty(&q->list))
                q = NULL;
in sigqueue_free() smells like it's asking for races.  Sigh...