Re: [PATCH v3] nfsd: remove long-standing revoked delegations by force

[Chuck Lever <chuck.lever@oracle.com>]

On 9/4/25 7:48 PM, Li Lingfeng wrote:
> When file access conflicts occur between clients, the server recalls
> delegations. If the client holding delegation fails to return it after
> a recall, nfs4_laundromat adds the delegation to cl_revoked list.
> This causes subsequent SEQUENCE operations to set the
> SEQ4_STATUS_RECALLABLE_STATE_REVOKED flag, forcing the client to
> validate all delegations and return the revoked one.
> 
> However, if the client fails to return the delegation like this:
> nfs4_laundromat                       nfsd4_delegreturn
>  unhash_delegation_locked
>  list_add // add dp to reaplist
>           // by dl_recall_lru
>  list_del_init // delete dp from
>                // reaplist
>                                        destroy_delegation
>                                         unhash_delegation_locked
>                                          // do nothing but return false
>  revoke_delegation
>  list_add // add dp to cl_revoked
>           // by dl_recall_lru
> 
> The delegation will remain in the server's cl_revoked list while the
> client marks it revoked and won't find it upon detecting
> SEQ4_STATUS_RECALLABLE_STATE_REVOKED.
> This leads to a loop:
> the server persistently sets SEQ4_STATUS_RECALLABLE_STATE_REVOKED, and the
> client repeatedly tests all delegations, severely impacting performance
> when numerous delegations exist.
> 
> Since abnormal delegations are removed from flc_lease via nfs4_laundromat
> --> revoke_delegation --> destroy_unhashed_deleg -->
> nfs4_unlock_deleg_lease --> kernel_setlease, and do not block new open
> requests indefinitely, retaining such a delegation on the server is
> unnecessary.
> 
> Fixes: 3bd64a5ba171 ("nfsd4: implement SEQ4_STATUS_RECALLABLE_STATE_REVOKED")
> Reported-by: Zhang Jian <zhangjian496@huawei.com>
> Closes: https://lore.kernel.org/all/ff8debe9-6877-4cf7-ba29-fc98eae0ffa0@huawei.com/
> Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
> Reviewed-by: Jeff Layton <jlayton@kernel.org>
> ---
>   Changes in v2:
>   1) Set SC_STATUS_CLOSED unconditionally in destroy_delegation();
>   2) Determine whether to remove the delegation based on SC_STATUS_CLOSED,
>      rather than by timeout;
>   3) Modify the commit message.
> 
>   Changes in v3:
>   1) Move variables used for traversal inside the if statement;
>   2) Add a comment to explain why we have to do this;
>   3) Move the second check of cl_revoked inside the if statement of
>      the first check.
>  fs/nfsd/nfs4state.c | 35 +++++++++++++++++++++++++++++++++--
>  1 file changed, 33 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index 88c347957da5..20fae3449af6 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -1336,6 +1336,11 @@ static void destroy_delegation(struct nfs4_delegation *dp)
>  
>  	spin_lock(&state_lock);
>  	unhashed = unhash_delegation_locked(dp, SC_STATUS_CLOSED);
> +	/*
> +	 * Unconditionally set SC_STATUS_CLOSED, regardless of whether the
> +	 * delegation is hashed, to mark the current delegation as invalid.
> +	 */
> +	dp->dl_stid.sc_status |= SC_STATUS_CLOSED;
>  	spin_unlock(&state_lock);
>  	if (unhashed)
>  		destroy_unhashed_deleg(dp);
> @@ -4470,8 +4475,34 @@ nfsd4_sequence(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
>  	default:
>  		seq->status_flags = 0;
>  	}
> -	if (!list_empty(&clp->cl_revoked))
> -		seq->status_flags |= SEQ4_STATUS_RECALLABLE_STATE_REVOKED;
> +	if (!list_empty(&clp->cl_revoked)) {
> +		struct list_head *pos, *next;
> +		struct nfs4_delegation *dp;
> +
> +		/*
> +		 * Concurrent nfs4_laundromat() and nfsd4_delegreturn()
> +		 * may add a delegation to cl_revoked even after the
> +		 * client has returned it, causing persistent
> +		 * SEQ4_STATUS_RECALLABLE_STATE_REVOKED, disrupting normal
> +		 * operations.
> +		 * Remove delegations with SC_STATUS_CLOSED from cl_revoked
> +		 * to resolve.
> +		 */
> +		spin_lock(&clp->cl_lock);
> +		list_for_each_safe(pos, next, &clp->cl_revoked) {
> +			dp = list_entry(pos, struct nfs4_delegation, dl_recall_lru);
> +			if (dp->dl_stid.sc_status & SC_STATUS_CLOSED) {
> +				list_del_init(&dp->dl_recall_lru);
> +				spin_unlock(&clp->cl_lock);

Does unlocking cl_lock here allow another CPU to free the object
that @next is pointing to? That pointer address would then be
dereferenced on the next loop iteration.

Might be better to stuff dp onto a local list, then "put" all
the items on that list once this loop has terminated and cl_lock
has been released.


> +				nfs4_put_stid(&dp->dl_stid);
> +				spin_lock(&clp->cl_lock);
> +			}
> +		}
> +		spin_unlock(&clp->cl_lock);
> +
> +		if (!list_empty(&clp->cl_revoked))
> +			seq->status_flags |= SEQ4_STATUS_RECALLABLE_STATE_REVOKED;
> +	}
>  	if (atomic_read(&clp->cl_admin_revoked))
>  		seq->status_flags |= SEQ4_STATUS_ADMIN_STATE_REVOKED;
>  	trace_nfsd_seq4_status(rqstp, seq);


-- 
Chuck Lever