Messages in this thread | | | Date | Tue, 25 Sep 2012 07:05:59 +0200 | From | Herbert Poetzl <> | Subject | Re: [2.6.38-3.x] [BUG] soft lockup - CPU#X stuck for 23s! (vfs, autofs, vserver) |
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On Mon, Sep 24, 2012 at 11:17:42AM -0700, Eric W. Biederman wrote: > Herbert Poetzl <herbert@13thfloor.at> writes:
>> On Mon, Sep 24, 2012 at 07:23:55AM +0200, Paweł Sikora wrote: >>> On Sunday 23 of September 2012 18:10:30 Linus Torvalds wrote: >>>> On Sat, Sep 22, 2012 at 11:09 PM, Paweł Sikora <pluto@pld-linux.org> 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 = current->fs->root; >> root_mnt = real_mount(root.mnt); >> point = root.dentry;
>> while ((mnt != mnt->mnt_parent) && (mnt != root_mnt)) { >> point = mnt->mnt_mountpoint; >> mnt = mnt->mnt_parent; >> }
>> ret = (mnt == root_mnt) && is_subdir(point, root.dentry); >> 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!)
> 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. Paweł 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.
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