On Thu, Jan 22, 2015 at 10:55:42PM -0800, 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.)
And I did finally figure out how this can happen. Please see below
for an updated patch with this information recorded in the commit log.
Sasha, I am impressed -- your testing not only located a true RCU bug,
but an RCU bug that can happen on a uniprocessor! ;-)
As far as I know, the bug is harmless apart from the splat, but still...
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,
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.
For this problem to occur, we need rcu_preempt_data.passed_quiesce equal
to true and current->rcu_read_unlock_special.b.need_qs also equal to true.
This condition can occur as follows:
1. CPU 0 is aware of the current preemptible RCU grace period,
but has not yet passed through a quiescent state. Among other
things, this means that rcu_preempt_data.passed_quiesce is false.
2. Task A running on CPU 0 enters a preemptible RCU read-side
critical section.
3. CPU 0 takes a scheduling-clock interrupt, which notices the
RCU read-side critical section and the need for a quiescent state,
and thus sets current->rcu_read_unlock_special.b.need_qs to true.
4. Task A is preempted, enters the scheduler, eventually invoking
rcu_preempt_note_context_switch() which in turn invokes
rcu_preempt_qs().
Because rcu_preempt_data.passed_quiesce is false,
control enters the body of the "if" statement, which sets
rcu_preempt_data.passed_quiesce to true.
5. At this point, CPU 0 takes an interrupt. The interrupt
handler contains an RCU read-side critical section, and
the rcu_read_unlock() notes that current->rcu_read_unlock_special
is nonzero, and thus invokes rcu_read_unlock_special().
6. Once in rcu_read_unlock_special(), the fact that
current->rcu_read_unlock_special.b.need_qs is true becomes
apparent, so rcu_read_unlock_special() invokes rcu_preempt_qs().
Recursively, given that we interrupted out of that same
function in the preceding step.
7. Because rcu_preempt_data.passed_quiesce is now true,
rcu_preempt_qs() does nothing, and simply returns.
8. Upon return to rcu_read_unlock_special(), it is noted that
current->rcu_read_unlock_special is still nonzero (because
the interrupted rcu_preempt_qs() had not yet gotten around
to clearing current->rcu_read_unlock_special.b.need_qs).
9. Execution proceeds to the WARN_ON_ONCE(), which notes that
we are in an interrupt handler and thus duly splats.
The solution, as noted above, is to make rcu_read_unlock_special()
clear out current->rcu_read_unlock_special.b.need_qs after calling
rcu_preempt_qs(). The interrupted rcu_preempt_qs() will clear it again,
but this is harmless. The worst that happens is that we clobber another
attempt to set this field, but this is not a problem because we just
got done reporting a quiescent state.
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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