linux-kernel - Re: [PATCH v2] nfsd: hold a lighter-weight client reference over CB_RECALL

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Date: Tue, 9 Apr 2024 13:32:07 -0400
From: Chuck Lever <chuck.lever@...cle.com>
To: Rik Theys <Rik.Theys@...t.kuleuven.be>
Cc: Jeff Layton <jlayton@...nel.org>, Neil Brown <neilb@...e.de>,
        Olga Kornievskaia <kolga@...app.com>, Dai Ngo <Dai.Ngo@...cle.com>,
        Tom Talpey <tom@...pey.com>, Vladimir Benes <vbenes@...hat.com>,
        linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] nfsd: hold a lighter-weight client reference over
 CB_RECALL_ANY

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@...hat.com>
> > > Signed-off-by: Jeff Layton<jlayton@...nel.org>
> > 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