Messages in this thread Patch in this message | | | Date | Thu, 22 Jan 2015 22:55:42 -0800 | From | "Paul E. McKenney" <> | Subject | Re: rcu, sched: WARNING: CPU: 30 PID: 23771 at kernel/rcu/tree_plugin.h:337 rcu_read_unlock_special+0x369/0x550() |
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On Thu, Jan 22, 2015 at 11:05:45PM -0500, Sasha Levin wrote: > On 01/22/2015 11:02 PM, Sasha Levin wrote: > > On 01/22/2015 10:51 PM, Paul E. McKenney wrote: > >> On Thu, Jan 22, 2015 at 10:29:01PM -0500, Sasha Levin wrote: > >>>> On 01/21/2015 07:43 PM, Paul E. McKenney wrote: > >>>>>> On Wed, Jan 21, 2015 at 10:44:57AM -0500, Sasha Levin wrote: > >>>>>>>> On 01/20/2015 09:57 PM, Paul E. McKenney wrote: > >>>>>>>>>>>>>> So RCU believes that an RCU read-side critical section that ended within > >>>>>>>>>>>>>>>>>> an interrupt handler (in this case, an hrtimer) somehow got preempted. > >>>>>>>>>>>>>>>>>> Which is not supposed to happen. > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> Do you have CONFIG_PROVE_RCU enabled? If not, could you please enable it > >>>>>>>>>>>>>>>>>> and retry? > >>>>>>>>>>>>>> > >>>>>>>>>>>>>> I did have CONFIG_PROVE_RCU, and didn't see anything else besides what I pasted here. > >>>>>>>>>> OK, fair enough. I do have a stack of RCU CPU stall-warning changes on > >>>>>>>>>> their way in, please see v3.19-rc1..630181c4a915 in -rcu, which is at: > >>>>>>>>>> > >>>>>>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git > >>>>>>>>>> > >>>>>>>>>> These handle the problems that Dave Jones, yourself, and a few others > >>>>>>>>>> located this past December. Could you please give them a spin? > >>>>>>>> > >>>>>>>> They seem to be a part of -next already, so this testing already includes them. > >>>>>>>> > >>>>>>>> I seem to be getting them about once a day, anything I can add to debug it? > >>>>>> > >>>>>> Could you please try reproducing with the following patch? > >>>> > >>>> Yes, and I've got mixed results. It reproduced, and all I got was: > >>>> > >>>> [ 717.645572] =============================== > >>>> [ 717.645572] [ INFO: suspicious RCU usage. ] > >>>> [ 717.645572] 3.19.0-rc5-next-20150121-sasha-00064-g3c37e35-dirty #1809 Tainted: G W > >>>> [ 717.645572] ------------------------------- > >>>> [ 717.645572] kernel/rcu/tree_plugin.h:337 rcu_read_unlock() from irq or softirq with blocking in critical section!!! > >>>> [ 717.645572] ! > >>>> [ 717.645572] > >>>> [ 717.645572] other info that might help us debug this: > >>>> [ 717.645572] > >>>> [ 717.645572] > >>>> [ 717.645572] rcu_scheduler_active = 1, debug_locks = 1 > >>>> [ 717.645572] 3 locks held by trinity-c29/16497: > >>>> [ 717.645572] #0: (&sb->s_type->i_mutex_key){+.+.+.}, at: [<ffffffff81bec373>] lookup_slow+0xd3/0x420 > >>>> [ 717.645572] #1: > >>>> [hang] > >>>> > >>>> So the rest of the locks/stack trace didn't get printed, nor the pr_alert() which > >>>> should follow that. > >>>> > >>>> I've removed the lockdep call and will re-run it. > >> Thank you! You are keeping the pr_alert(), correct? > > > > Yup, just the lockdep call goes away. > > Okay, this reproduced faster than I anticipated: > > [ 786.160131] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1) > [ 786.239513] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1) > [ 786.240503] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1) > [ 786.242575] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1) > [ 786.243565] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1) > [ 786.243565] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1) > [ 786.243565] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1) > [ 786.243565] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1) > [ 786.243565] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1) > > It seems like the WARN_ON_ONCE was hiding the fact it actually got hit couple > of times in a very short interval. Maybe that would also explain lockdep crapping > itself.
OK, that was what I thought was the situation. I have not yet fully worked out how RCU gets into that state, but in the meantime, here is a patch that should prevent the splats. (It requires a subtle interaction of quiescent-state detection and the scheduling-clock interrupt.)
Thanx, Paul
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rcu: Clear need_qs flag to prevent splat
If the scheduling-clock interrupt sets the current tasks need_qs flag, but if the current CPU passes through a quiescent state in the meantime, then rcu_preempt_qs() will fail to clear the need_qs flag, which can fool RCU into thinking that additional rcu_read_unlock_special() processing is needed. This commit therefore clears the need_qs flag before checking for additional processing.
Reported-by: Sasha Levin <sasha.levin@oracle.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 8669de884445..ec99dc16aa38 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -322,6 +322,7 @@ void rcu_read_unlock_special(struct task_struct *t) special = t->rcu_read_unlock_special; if (special.b.need_qs) { rcu_preempt_qs(); + t->rcu_read_unlock_special.need_qs = false; if (!t->rcu_read_unlock_special.s) { local_irq_restore(flags); return;
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