On Tue, May 27, 2014 at 02:18:36PM -0400, Steven Rostedt wrote: > [ moving this to LKML from linux-rt-users, as that's where it should be ] > > On Sat, 17 May 2014 05:36:59 +0200 > Mike Galbraith <[email protected]> wrote: > > > 3.14-rt being build with a non-rt config is unlikely, but.. > > > > >From 60e69eed85bb7b5198ef70643b5895c26ad76ef7 Mon Sep 17 00:00:00 2001 > > From: Mike Galbraith <[email protected]> > > Date: Mon, 7 Apr 2014 10:55:15 +0200 > > Subject: [PATCH] sched/numa: Fix task_numa_free() lockdep splat > > > > Sasha reported that lockdep claims that the following commit: > > made numa_group.lock interrupt unsafe: > > > > 156654f491dd ("sched/numa: Move task_numa_free() to __put_task_struct()") > > > > While I don't see how that could be, given the commit in question moved > > task_numa_free() from one irq enabled region to another, the below does > > make both gripes and lockups upon gripe with numa=fake=4 go away. > > It wasn't the irqs that was causing the lockdep splat, but the > softirqs. You moved it into __put_task_struct() which is called as a > rcu callback that gets called from soft irqs. So yes, you need to > prevent softirqs from happening whenever you take the lock. > spin_lock_irq() is a bigger hammer than needed. The patch below should > be good enough. > > I kept the double_lock_irq() as there is no double_lock_bh(). Should we > bother to make one?
Nope, its really IRQs.
do_exit()
exit_itimers()
itimer_delete()
spin_lock_irqsave(&timer->it_lock, &flags);
timer_delete_hook(timer);
kc->timer_del(timer) := posix_cpu_timer_del()
put_task_struct()
__put_task_struct()
task_numa_free()
spin_lock(&grp->lock);
Which nests the grp->lock inside the timer->it_lock, and where the
timer->it_lock is IRQ-safe, the grp->lock is not.
This allows for IRQ deadlocks.
pgpcE2sda5W1L.pgp
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