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 ...
best,
Herbert
> BR,
> Paweł.
> ps).
> i'm adding Herbert to CC.
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