[PATCH 6.12.y 4/5] NFSD: Count the delegations held by each client

[Sasha Levin <sashal@kernel.org>]

From: Chuck Lever <cel@kernel.org>

[ Upstream commit 5d5a6e996e7c36bc2f7b035743c16f2bf03a227d ]

struct nfs4_client records the delegations it holds on cl_delegations
but keeps no count of them. deleg_reaper() walks nn->client_lru under
nn->client_lock, but cl_delegations is serialized by nn->deleg_lock,
which nests outside nn->client_lock. A caller there cannot take
nn->deleg_lock to count the list. The cost tells against the walk as
well: an O(n) count per client, on a pass that already visits every
client.

Add cl_deleg_count, maintained at the two sites that mutate
cl_delegations. Both hold nn->deleg_lock, so the counter is already
serialized against itself and needs no atomic of its own. The decrement
sits below the delegation_hashed() test, next to the list_del_init it
pairs with, so it runs only when the delegation really leaves the list.

A reader that holds only nn->client_lock is not synchronized against
either update site, so it can see a count that does not match the
list. Such a reader marks the access with data_race() and may not
depend on the value for correctness.

No functional change.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260817-recall-any-keep-count-v5-2-3b2cffce701e@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>
Stable-dep-of: 9af39c0773ef ("NFSD: Send a meaningful CB_RECALL_ANY keep count")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/nfsd/nfs4state.c | 2 ++
 fs/nfsd/state.h     | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index d2878bf1e7181..43755bce08f95 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -1420,6 +1420,7 @@ hash_delegation_locked(struct nfs4_delegation *dp, struct nfs4_file *fp)
 	dp->dl_stid.sc_type = SC_TYPE_DELEG;
 	list_add(&dp->dl_perfile, &fp->fi_delegations);
 	list_add(&dp->dl_perclnt, &clp->cl_delegations);
+	clp->cl_deleg_count++;
 	return 0;
 }
 
@@ -1451,6 +1452,7 @@ unhash_delegation_locked(struct nfs4_delegation *dp, unsigned short statusmask)
 	++dp->dl_time;
 	spin_lock(&fp->fi_lock);
 	list_del_init(&dp->dl_perclnt);
+	dp->dl_stid.sc_client->cl_deleg_count--;
 	list_del_init(&dp->dl_recall_lru);
 	list_del_init(&dp->dl_perfile);
 	spin_unlock(&fp->fi_lock);
diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
index 4e0e36b81b203..1909d744d4ac5 100644
--- a/fs/nfsd/state.h
+++ b/fs/nfsd/state.h
@@ -457,6 +457,8 @@ struct nfs4_client {
 
 	unsigned int		cl_state;
 	atomic_t		cl_delegs_in_recall;
+	/* Length of cl_delegations, updated under nn->deleg_lock */
+	unsigned int		cl_deleg_count;
 
 	struct nfsd4_cb_recall_any	*cl_ra;
 	time64_t		cl_ra_time;
-- 
2.53.0