From a71f161a857117e8e0264deb7d14fff5c98adcf5 Mon Sep 17 00:00:00 2001 From: Jeff Layton Date: Thu, 11 Jun 2026 16:00:45 -0400 Subject: [PATCH] nfsd: add missing read barrier to rpc_status_get dumpit seqcount retry 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 counter check to order the field reads ahead of it, matching the barrier semantics of the standard seqcount read-side. The begin-side smp_load_acquire() already pairs with the smp_store_release() in nfsd_dispatch(); with the smp_rmb() now ordering the field reads, the retry check no longer needs acquire semantics and reads the counter with a plain READ_ONCE(), as read_seqcount_retry() does. Fixes: bd9d6a3efa97 ("NFSD: add rpc_status netlink support") Cc: stable@vger.kernel.org Assisted-by: Claude:claude-opus-4-8 Signed-off-by: Jeff Layton [ cel: Use READ_ONCE instead of smp_load_acquire() ] Link: https://patch.msgid.link/20260611-nfsd-testing-v2-2-5b90e276f2d9@kernel.org Signed-off-by: Chuck Lever --- fs/nfsd/nfsctl.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c index eedc2e6bc45a..38c7468906a1 100644 --- a/fs/nfsd/nfsctl.c +++ b/fs/nfsd/nfsctl.c @@ -1576,11 +1576,14 @@ 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. + * Read-side load-load fence: order the field reads + * above before the counter re-read below, mirroring + * the smp_rmb() in the standard seqcount retry. The + * begin-side smp_load_acquire() above pairs with the + * smp_store_release() in nfsd_dispatch(). */ - if (smp_load_acquire(&rqstp->rq_status_counter) != - status_counter) + smp_rmb(); + if (READ_ONCE(rqstp->rq_status_counter) != status_counter) continue; ret = nfsd_genl_rpc_status_compose_msg(skb, cb,