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

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Message-ID: <51fdd36542d253e92153ed3bc28f69866821735c.camel@kernel.org>
Date: Fri, 05 Apr 2024 13:08:02 -0400
From: Jeff Layton <jlayton@...nel.org>
To: Chuck Lever <chuck.lever@...cle.com>, Neil Brown <neilb@...e.de>, Olga
 Kornievskaia <kolga@...app.com>, Dai Ngo <Dai.Ngo@...cle.com>, Tom Talpey
 <tom@...pey.com>
Cc: Vladimir Benes <vbenes@...hat.com>, linux-nfs@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nfsd: hold a lighter-weight client reference over
 CB_RECALL_ANY

On Fri, 2024-04-05 at 13:00 -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, this has the effect
> of preventing the client from being unhashed.
> 
> 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 also can't connect because
> the old client can't be unhashed.
> 
> The job is only holding a reference to the client so it can clear a flag
> in the client after it completes. This patch attempts to alleviate this
> by having the job 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 too.
> 

This should probably also have:

    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>
> ---
> This patch seems to break the livelock in my testing. It's not the
> prettiest fix, but it's narrowly targeted and should be appropriate for
> 6.9-rc. Longer term, I think we need to rework how the nfs4_clients
> refcounts are managed, but that's a larger project.
> 
> Many thanks to Vladimir for all his help with tracking this down!
> ---
>  fs/nfsd/nfs4state.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index 5fcd93f7cb8c..4311d608a297 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -3042,12 +3042,9 @@ static void
>  nfsd4_cb_recall_any_release(struct nfsd4_callback *cb)
>  {
>  	struct nfs4_client *clp = cb->cb_clp;
> -	struct nfsd_net *nn = net_generic(clp->net, nfsd_net_id);
>  
> -	spin_lock(&nn->client_lock);
>  	clear_bit(NFSD4_CLIENT_CB_RECALL_ANY, &clp->cl_flags);
> -	put_client_renew_locked(clp);
> -	spin_unlock(&nn->client_lock);
> +	drop_client(clp);
>  }
>  
>  static int
> @@ -6613,10 +6610,12 @@ deleg_reaper(struct nfsd_net *nn)
>  				clp->cl_ra_time < 5)) {
>  			continue;
>  		}
> -		list_add(&clp->cl_ra_cblist, &cblist);
>  
>  		/* release in nfsd4_cb_recall_any_release */
> -		atomic_inc(&clp->cl_rpc_users);
> +		if (!kref_get_unless_zero(&clp->cl_nfsdfs.cl_ref))
> +			continue;
> +
> +		list_add(&clp->cl_ra_cblist, &cblist);
>  		set_bit(NFSD4_CLIENT_CB_RECALL_ANY, &clp->cl_flags);
>  		clp->cl_ra_time = ktime_get_boottime_seconds();
>  	}
> 
> ---
> base-commit: 05258a0a69b3c5d2c003f818702c0a52b6fea861
> change-id: 20240405-rhel-31513-028ab6f14252
> 
> Best regards,

-- 
Jeff Layton <jlayton@...nel.org>