From mboxrd@z Thu Jan 1 00:00:00 1970 From: Herbert Poetzl Subject: Re: [2.6.38-3.x] [BUG] soft lockup - CPU#X stuck for 23s! (vfs, autofs, vserver) Date: Thu, 15 Nov 2012 20:22:27 +0100 Message-ID: <20121115192226.GC650@MAIL.13thfloor.at> References: <5092540.GORQ1kUuNX@localhost> <87sja7uvy1.fsf@xmission.com> <20120925050558.GA14685@MAIL.13thfloor.at> <3506450.k3Q223DJQc@localhost> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: "Eric W. Biederman" , Linus Torvalds , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, arekm@pld-linux.org, baggins@pld-linux.org, Daniel Hokka Zakrisson To: =?utf-8?B?UGF3ZcWC?= Sikora Return-path: Received: from MAIL.13thfloor.at ([213.145.232.33]:55320 "EHLO MAIL.13thfloor.at" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1768799Ab2KOTkf (ORCPT ); Thu, 15 Nov 2012 14:40:35 -0500 Content-Disposition: inline In-Reply-To: <3506450.k3Q223DJQc@localhost> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Thu, Nov 15, 2012 at 07:48:10PM +0100, Pawe=C5=82 Sikora wrote: > On Tuesday 25 of September 2012 07:05:59 Herbert Poetzl wrote: >> On Mon, Sep 24, 2012 at 11:17:42AM -0700, Eric W. Biederman wrote: >>> Herbert Poetzl writes: >>>> On Mon, Sep 24, 2012 at 07:23:55AM +0200, Pawe=C5=82 Sikora wrote: >>>>> On Sunday 23 of September 2012 18:10:30 Linus Torvalds wrote: >>>>>> On Sat, Sep 22, 2012 at 11:09 PM, Pawe=C5=82 Sikora wrote: >>>>>>> br_read_lock(vfsmount_lock); >>>>>> The vfsmount_lock is a "local-global" lock, where a read-lock >>>>>> is rather cheap and takes just a per-cpu lock, but the >>>>>> downside is that a write-lock is *very* expensive, and can >>>>>> cause serious trouble. >>>>>> And the write lock is taken by the [un]mount() paths. Do *not* >>>>>> do crazy things. If you do some insane "unmount and remount >>>>>> autofs" on a 1s granularity, you're doing insane things. >>>>>> Why do you have that 1s timeout? Insane. >>>>> 1s unmount timeout is *only* for fast bug reproduction (in few >>>>> seconds after opteron startup) and testing potential patches. >>>>> normally with 60s timeout it happens in few minutes..hours >>>>> (depends on machine i/o+cpu load) and makes server unusable >>>>> (permament soft-lockup). >>>>> can we redesign vserver's mnt_is_reachable() for better locking >>>>> to avoid total soft-lockup? >>>> currently we do: >>>> br_read_lock(&vfsmount_lock); >>>> root =3D current->fs->root; >>>> root_mnt =3D real_mount(root.mnt); >>>> point =3D root.dentry; >>>> while ((mnt !=3D mnt->mnt_parent) && (mnt !=3D root_mnt)) = { >>>> point =3D mnt->mnt_mountpoint; >>>> mnt =3D mnt->mnt_parent; >>>> } >>>> ret =3D (mnt =3D=3D root_mnt) && is_subdir(point, root.den= try); >>>> br_read_unlock(&vfsmount_lock); >>>> and we have been considering to move the br_read_unlock() >>>> right before the is_subdir() call >>>> if there are any suggestions how to achieve the same >>>> with less locking I'm all ears ... >>> Herbert, why do you need to filter the mounts that show up in a >>> mount namespace at all? >> that is actually a really good question! >>> I would think a far more performant and simpler solution would >>> be to just use mount namespaces without unwanted mounts. >> we had this mechanism for many years, long before the >> mount namespaces existed, and I vaguely remember that >> early versions didn't get the proc entries right either >> I took a quick look at the code and I think we can drop >> the mnt_is_reachable() check and/or make it conditional >> on setups without a mount namespace in place in the near >> future (thanks for the input, really appreciated!) > Hi, > Herbert, can i just drop this mnt_is_reachable() method > from vserver patch? this issue hasn't been solved for > several months now. i can live without this problematic > security-through-obscurity feature on my heavy loaded=20 > machines. sure, if you are aware of the implications, you can simply remove the check ... best, Herbert >>> I'd like to blame this on the silly rcu_barrier in >>> deactivate_locked_super that should really be in the module >>> remove path, but that happens after we drop the br_write_lock. >>> The kernel take br_read_lock(&vfs_mount_lokck) during every rcu >>> path lookup so mnt_is_reachable isn't particular crazy just for >>> taking the lock. >>> I am with Linus on this one. Pawe=C5=82 even 60s for your mount >>> timeout looks too short for your workload. All of the readers >>> that take br_read_lock(&vfsmount_lock) seem to be showing up in >>> your oops. The only thing that seems to make sense is you have >>> a lot of unmount activity running back to back, keeping the >>> lock write held. >>> The only other possible culprit I can see is that it looks like >>> mnt_is_reachable changes reading /proc/mounts to be something >>> worse than linear in the number of mounts and reading /proc/mounts >>> starts taking the vfsmount_lock. All minor things but when you >>> are pushing things hard they look like things that would add up. >>> Eric -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel= " in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html