Messages in this thread |  | | Date | Sat, 22 Mar 2008 17:55:53 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] fix rcu_try_flip_waitack_needed() to prevent grace-period stall |
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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. */ > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > the body of a message to majordomo@vger.kernel.org > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at http://www.tux.org/lkml/ >
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