Re: [PATCH v2] nfsd: hold a lighter-weight client reference over CB_RECALL_ANY

[Chuck Lever]

On Tue, Apr 09, 2024 at 07:24:17PM +0200, Rik Theys wrote:
> Hi,
> 
> On 4/5/24 20:07, Chuck Lever wrote:
> > On Fri, Apr 05, 2024 at 01:56:18PM -0400, Jeff Layton wrote:
> > > Currently the CB_RECALL_ANY job takes a cl_rpc_users reference to the
> > > client. While a callback job is technically an RPC that counter is
> > > really more for client-driven RPCs, and this has the effect of
> > > preventing the client from being unhashed until the callback completes.
> > > 
> > > If nfsd decides to send a CB_RECALL_ANY just as the client reboots, we
> > > can end up in a situation where the callback can't complete on the (now
> > > dead) callback channel, but the new client can't connect because the old
> > > client can't be unhashed. This usually manifests as a NFS4ERR_DELAY
> > > return on the CREATE_SESSION operation.
> > > 
> > > The job is only holding a reference to the client so it can clear a flag
> > > in the after the RPC completes. Fix this by having CB_RECALL_ANY instead
> > > hold a reference to the cl_nfsdfs.cl_ref. Typically we only take that
> > > sort of reference when dealing with the nfsdfs info files, but it should
> > > work appropriately here to ensure that the nfs4_client doesn't
> > > disappear.
> > > 
> > > Fixes: 44df6f439a17 ("NFSD: add delegation reaper to react to low memory condition")
> > > Reported-by: Vladimir Benes<vbenes@xxxxxxxxxx>
> > > Signed-off-by: Jeff Layton<jlayton@xxxxxxxxxx>
> > Applied to nfsd-fixes while waiting for review and testing. Thanks!
> > 
> Can this fix also be included in the 6.1.x LTS kernel? Given that "NFSD: add
> delegation reaper to react to low memory condition" was added to 6.1.81, it
> would be nice to have this fix in the 6.1 series.
> 
> This way it will also be picked up by Debian at some point (it seems they
> are upgrading to 6.1.82 for their next stable point release).

Thanks to the Fixes: tag in the commit's description, it will
probably appear in a later release of linux-6.1.y automatically.

If we don't see it the next few weeks, I will ping the stable
maintainers.


-- 
Chuck Lever