Herbert Poetzl <herbert@xxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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? I would think a far more performant and simpler
solution would be to just use mount namespaces without unwanted mounts.
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.
Eric
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
