Messages in this thread | | | Date | Tue, 11 Aug 2009 07:52:22 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [BUG] race of RCU vs NOHU |
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On Tue, Aug 11, 2009 at 12:56:53PM +0200, Martin Schwidefsky wrote: > On Mon, 10 Aug 2009 08:08:07 -0700 > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote: > > > On Mon, Aug 10, 2009 at 02:25:35PM +0200, Martin Schwidefsky wrote: > > > On Fri, 7 Aug 2009 07:29:57 -0700 > > > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote: > > > > > > > On Fri, Aug 07, 2009 at 03:15:29PM +0200, Martin Schwidefsky wrote: > > > > > Hi Paul, > > > > > I analysed a dump of a hanging 2.6.30 system and found what I think is > > > > > a bug of RCU vs NOHZ. There are a number of patches ontop of that > > > > > kernel but they should be independent of the bug. > > > > > > > > > > The systems has 4 cpus and uses classic RCU. cpus #0, #2 and #3 woke up > > > > > recently, cpu #1 has been sleeping for 5 minutes, but there is a pending > > > > > rcu batch. The timer wheel for cpu #1 is empty, it will continue to > > > > > sleep for NEXT_TIMER_MAX_DELTA ticks. > > > > > > > > Congratulations, Martin! You have exercised what to date has been a > > > > theoretical bug identified last year by Manfred Spraul. The fix is to > > > > switch from CONFIG_RCU_CLASSIC to CONFIG_RCU_TREE, which was added in > > > > 2.6.29. > > > > > > > > Of course, if you need to work with an old kernel version, you might > > > > still need a patch, perhaps for the various -stable versions. If so, > > > > please let me know -- otherwise, I will focus forward on CONFIG_RCU_TREE > > > > rather than backwards on CONFIG_RCU_CLASSIC. > > > > > > SLES11 is 2.6.27 and uses classic RCU. The not-so theoretical bug is > > > present there and I think it needs to be fixed :-/ > > > > I was afraid of that. ;-) > > > > Given that there are some other theoretical bugs in Classic RCU involving > > interrupts and CONFIG_NO_HZ, would backporting CONFIG_TREE_RCU make more > > sense than playing whack-a-mole on Classic RCU bugs? > > Fine with me but I don't know if SuSE/Novell is willing to accept such a > big change for an existing distribution. I've put Ihno and Greg on Cc.
Good point! While they are thinking about the tradeoff between whack-a-mole on Classic RCU and backporting CONFIG_TREE_RCU, if I was to send you a patch backporting CONFIG_TREE_RCU, to exactly which kernel version(s) should I backport it to?
Thanx, Paul
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