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SubjectRe: [PATCH] fix rcu_try_flip_waitack_needed() to prevent grace-period stall
On Sat, Mar 22, 2008 at 06:43:48PM +0100, Peter Zijlstra wrote:
> On Fri, 2008-03-21 at 13:38 -0700, Paul E. McKenney wrote:
> > The comment was correct -- need to make the code match the comment.
> > Without this patch, if a CPU goes dynticks idle (and stays there forever)
> > in just the right phase of preemptible-RCU grace-period processing,
> > grace periods stall. The offending sequence of events (courtesy
> > of Promela/spin, at least after I got the liveness criterion coded
> > correctly...) is as follows:
> >
> > o CPU 0 is in dynticks-idle mode. Its dynticks_progress_counter
> > is (say) 10.
> >
> > o CPU 0 takes an interrupt, so rcu_irq_enter() increments CPU 0's
> > dynticks_progress_counter to 11.
> >
> > o CPU 1 is doing RCU grace-period processing in rcu_try_flip_idle(),
> > sees rcu_pending(), so invokes dyntick_save_progress_counter(),
> > which in turn takes a snapshot of CPU 0's dynticks_progress_counter
> > into CPU 0's rcu_dyntick_snapshot -- now set to 11. CPU 1 then
> > updates the RCU grace-period state to rcu_try_flip_waitack().
> >
> > o CPU 0 returns from its interrupt, so rcu_irq_exit() increments
> > CPU 0's dynticks_progress_counter to 12.
> >
> > o CPU 1 later invokes rcu_try_flip_waitack(), which notices that
> > CPU 0 has not yet responded, and hence in turn invokes
> > rcu_try_flip_waitack_needed(). This function examines the
> > state of CPU 0's dynticks_progress_counter and rcu_dyntick_snapshot
> > variables, which it copies to curr (== 12) and snap (== 11),
> > respectively.
> >
> > Because curr!=snap, the first condition fails.
> >
> > Because curr-snap is only 1 and snap is odd, the second
> > condition fails.
> >
> > rcu_try_flip_waitack_needed() therefore incorrectly concludes
> > that it must wait for CPU 0 to explicitly acknowledge the
> > counter flip.
> >
> > o CPU 0 remains forever in dynticks-idle mode, never taking
> > any more hardware interrupts or any NMIs, and never running
> > any more tasks. (Of course, -something- will usually eventually
> > happen, which might be why we haven't seen this one in the
> > wild. Still should be fixed!)
> >
> > Therefore the grace period never ends. Fix is to make the code match
> > the comment, as shown below. With this fix, the above scenario
> > would be satisfied with curr being even, and allow the grace period
> > to proceed.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> Paul, should this go upstream ASAP?

Given that any activity (task wakeup, interrupt, NMI) on the offending CPU
gets things going again, I have a hard time labeling it as super urgent.
So I could argue for it being added to the last release candidate,
but not to the final release itself.

Seem reasonable?

Thanx, Paul

> > ---
> >
> > rcupreempt.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff -urpNa -X dontdiff linux-2.6.25-rc6/kernel/rcupreempt.c linux-2.6.25-rc6-rcunohz-if/kernel/rcupreempt.c
> > --- linux-2.6.25-rc6/kernel/rcupreempt.c 2008-03-16 17:45:17.000000000 -0700
> > +++ linux-2.6.25-rc6-rcunohz-if/kernel/rcupreempt.c 2008-03-18 20:27:47.000000000 -0700
> > @@ -569,7 +569,7 @@ rcu_try_flip_waitack_needed(int cpu)
> > * that this CPU already acknowledged the counter.
> > */
> >
> > - if ((curr - snap) > 2 || (snap & 0x1) == 0)
> > + if ((curr - snap) > 2 || (curr & 0x1) == 0)
> > return 0;
> >
> > /* We need this CPU to explicitly acknowledge the counter flip. */
> > --
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