Messages in this thread |  | | Subject | Re: [PATCH] fix rcu_try_flip_waitack_needed() to prevent grace-period stall | From | Peter Zijlstra <> | Date | Sat, 22 Mar 2008 18:43:48 +0100 |
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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?
> --- > > 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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