On Fri, Jan 23, 2015 at 04:41:12PM +0800, Lai Jiangshan wrote:
> On 01/23/2015 02:55 PM, Paul E. McKenney wrote:
> > 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
> >
> > ------------------------------------------------------------------------
> >
> > 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,
>
> How does the above things happen?
> "the scheduling-clock interrupt" is local-irq-disabled.
The scheduling-clock interrupt just sets things up. The remainder of the
failure scenario happens after this interrupt returns. See my earlier
email for the whole sad story. ;-)
Thanx, Paul
> > 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.le...@oracle.com>
> > Signed-off-by: Paul E. McKenney <paul...@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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