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

[Raghavendra Gowdappa]

----- Original Message -----
> From: "Raghavendra Gowdappa" <rgowdapp at redhat.com>
> To: "Nithya Balachandran" <nbalacha at redhat.com>
> Cc: gluster-devel at gluster.org
> Sent: Tuesday, May 17, 2016 2:40:32 PM
> Subject: Re: [Gluster-devel] 'mv' of ./tests/bugs/posix/bug-1113960.t causes 100% CPU
> 
> 
> 
> ----- 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. 

Please refer to bz 1335373 [1] for more details.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1335373

> 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
> > 
> > _______________________________________________
> > Gluster-devel mailing list
> > Gluster-devel at gluster.org
> > http://www.gluster.org/mailman/listinfo/gluster-devel
> > 
> > 
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