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

[Raghavendra Gowdappa]

----- Original Message -----
> From: "Nithya Balachandran" <nbalacha at redhat.com>
> To: "Niels de Vos" <ndevos at redhat.com>
> Cc: gluster-devel at gluster.org
> Sent: Tuesday, May 17, 2016 2:25:20 PM
> Subject: Re: [Gluster-devel] 'mv' of ./tests/bugs/posix/bug-1113960.t causes 100% CPU
> 
> 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.

I think the stale dentry is caused by a racing lookup and rename. Apart from that, I don't know of any other reasons for stale dentries in inode table. "Dentry fop serializer" (DFS) [1], aims to solve these kind of races.

[1] http://review.gluster.org/14286

> 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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