[PATCH v2 02/21] nfsd: add missing read barrier to rpc_status_get dumpit seqcount retry

[Jeff Layton <jlayton@kernel.org>]

The hand-rolled seqcount-like protocol in nfsd_nl_rpc_status_get_dumpit()
is missing a read memory barrier (smp_rmb) before its second counter
check.  The standard kernel read_seqcount_retry() includes smp_rmb()
to ensure that all data reads complete before the counter is re-checked.

Without this barrier, on weakly-ordered architectures (ARM, POWER),
the CPU may reorder field reads past the second counter check, making
the retry logic ineffective: it could observe a consistent counter pair
while reading fields that have been concurrently modified by the writer.

Add smp_rmb() before the second smp_load_acquire() to match the
barrier semantics of the standard seqcount read-side.

Fixes: ac18892ea3f7 ("NFSD: add rpc_status netlink support")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/nfsd/nfsctl.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
index c06d25c06f06..a4b5b1467fe2 100644
--- a/fs/nfsd/nfsctl.c
+++ b/fs/nfsd/nfsctl.c
@@ -1576,9 +1576,11 @@ int nfsd_nl_rpc_status_get_dumpit(struct sk_buff *skb,
 #endif /* CONFIG_NFSD_V4 */
 
 			/*
-			 * Acquire rq_status_counter before reporting the rqst
-			 * fields to the user.
+			 * Ensure all field reads complete before re-checking
+			 * the status counter. Pairs with the smp_store_release
+			 * in nfsd_dispatch to form a seq-lock like protocol.
 			 */
+			smp_rmb();
 			if (smp_load_acquire(&rqstp->rq_status_counter) !=
 			    status_counter)
 				continue;

-- 
2.54.0