[Gluster-devel] 'mv' of ./tests/bugs/posix/bug-1113960.t causes 100% CPU

[Nithya Balachandran]

Hi,

I have looked into this on another system earlier and this is what I have
so far:

1. The test involves moving and renaming directories and files within those
dirs.
2. A rename dir operation failed on one subvol. So we have 3 subvols where
the directory has the new name and one where it has the old name.
3. Some operation - perhaps a revalidate - has added a dentry with the old
name to the inode . So there are now 2 dentries for the same inode for a
directory.
4. Renaming a file inside that directory calls an inode_link which end up
traversing the dentry list for each entry all the way up to the root in the
__foreach_ancestor_dentry function. If there are multiple deep directories
with the same problem in the path, this takes a very long time (hours)
because of the number of times the function is called.

I do not know why the rename dir failed. However, is the following a
correct/acceptable fix for the traversal issue?

1. A directory should never have more than one dentry
2. __foreach_ancestor_dentry uses the dentry list of the parent inode.
Parent inode will always be a directory.
3. Can we just take the first dentry in the list for the cycle check as we
are really only comparing inodes? In the scenarios I have tried, all the
dentries in the dentry_list always have the same inode.  This would prevent
the hang. If there is more than one dentry for a directory, flag an error
somehow.
4. Is there any chance that a dentry in the list can have a different
inode? If yes, that is a different problem and 3 does not apply.

It would work like this:


last_parent_inode = NULL;

        list_for_each_entry (each, &parent->dentry_list, inode_list) {

//Since we are only using the each->parent to check, stop if we have
already checked it

                if(each->parent != last_parent_inode) {
                     ret = __foreach_ancestor_dentry (each, per_dentry_fn,
data);
                     if (ret)
                             goto out;
                }
                last_parent_inode = each->parent;
        }


This would prevent the hang but leads to other issues which would exist in
the current code anyway - mainly, which dentry is the correct one and how
do we recover?


Regards,
Nithya

On Wed, May 11, 2016 at 7:09 PM, Niels de Vos <ndevos at redhat.com> wrote:

> Could someone look into this busy loop?
>   https://paste.fedoraproject.org/365207/29732171/raw/
>
> This was happening in a regression-test burn-in run, occupying a Jenkins
> slave for 2+ days:
>   https://build.gluster.org/job/regression-test-burn-in/936/
>   (run with commit f0ade919006b2581ae192f997a8ae5bacc2892af from master)
>
> A coredump of the mount process is available from here:
>   http://slave20.cloud.gluster.org/archived_builds/crash.tar.gz
>
> Thanks misc for reporting and gathering the debugging info.
> Niels
>
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>
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