On 06/18/2014 07:30 PM, Waiman Long wrote: > On 06/18/2014 07:27 PM, Chris Mason wrote: >> On 06/18/2014 07:19 PM, Waiman Long wrote: >>> On 06/18/2014 07:10 PM, Josef Bacik wrote: >>>> >>>> On 06/18/2014 03:47 PM, Waiman Long wrote: >>>>> On 06/18/2014 06:27 PM, Josef Bacik wrote: >>>>>> >>>>>> On 06/18/2014 03:17 PM, Waiman Long wrote: >>>>>>> On 06/18/2014 04:57 PM, Marc Dionne wrote: >>>>>>>> Hi, >>>>>>>> >>>>>>>> I've been seeing very reproducible soft lockups with 3.16-rc1 >>>>>>>> similar >>>>>>>> to what is reported here: >>>>>>>> https://urldefense.proofpoint.com/v1/url?u=http://marc.info/?l%3Dlinux-btrfs%26m%3D140290088532203%26w%3D2&k=ZVNjlDMF0FElm4dQtryO4A%3D%3D%0A&r=cKCbChRKsMpTX8ybrSkonQ%3D%3D%0A&m=aoagvtZMwVb16gh1HApZZL00I7eP50GurBpuEo3l%2B5g%3D%0A&s=c62558feb60a480bbb52802093de8c97b5e1f23d4100265b6120c8065bd99565 >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> , along with the >>>>>>>> occasional hard lockup, making it impossible to complete a parallel >>>>>>>> build on a btrfs filesystem for the package I work on. This was >>>>>>>> working fine just a few days before rc1. >>>>>>>> >>>>>>>> Bisecting brought me to the following commit: >>>>>>>> >>>>>>>> commit bd01ec1a13f9a327950c8e3080096446c7804753 >>>>>>>> Author: Waiman Long<waiman.l...@hp.com> >>>>>>>> Date: Mon Feb 3 13:18:57 2014 +0100 >>>>>>>> >>>>>>>> x86, locking/rwlocks: Enable qrwlocks on x86 >>>>>>>> >>>>>>>> And sure enough if I revert that commit on top of current mainline, >>>>>>>> I'm unable to reproduce the soft lockups and hangs. >>>>>>>> >>>>>>>> Marc >>>>>>> The queue rwlock is fair. As a result, recursive read_lock is not >>>>>>> allowed unless the task is in an interrupt context. Doing recursive >>>>>>> read_lock will hang the process when a write_lock happens >>>>>>> somewhere in >>>>>>> between. Are recursive read_lock being done in the btrfs code? >>>>>>> >>>>>> We walk down a tree and read lock each node as we walk down, is that >>>>>> what you mean? Or do you mean read_lock multiple times on the same >>>>>> lock in the same process, cause we definitely don't do that. Thanks, >>>>>> >>>>>> Josef >>>>> I meant recursively read_lock the same lock in a process. >>>> I take it back, we do actually do this in some cases. Thanks, >>>> >>>> Josef >>> This is what I thought when I looked at the looking code in btrfs. The >>> unlock code doesn't clear the lock_owner pid, this may cause the >>> lock_nested to be set incorrectly. >>> >>> Anyway, are you going to do something about it? >> Thanks for reporting this, we shouldn't be actually taking the lock >> recursively. Could you please try with lockdep enabled? If the problem >> goes away with lockdep on, I think I know what's causing it. Otherwise, >> lockdep should clue us in. >> >> -chris > > I am not sure if lockdep will report recursive read_lock as this is > possible in the past. If not, we certainly need to add that capability > to it. > > One more thing, I saw comment in btrfs tree locking code about taking a > read lock after taking a write (partial?) lock. That is not possible > with even with the old rwlock code.
With lockdep on, the clear_path_blocking function you're hitting softlockups in is different. Futjitsu hit a similar problem during quota rescans, and it goes away with lockdep on. I'm trying to nail down where we went wrong, but please try lockdep on. -chris -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
