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nfsd: add missing read barrier to rpc_status_get dumpit seqcount retry

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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: bd9d6a3efa ("NFSD: add rpc_status netlink support")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@kernel.org>
[ 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 <cel@kernel.org>
This commit is contained in:
Jeff Layton 2026-06-11 16:00:45 -04:00 committed by Chuck Lever
parent ae4c38555e
commit a71f161a85
1 changed files with 7 additions and 4 deletions
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11
fs/nfsd/nfsctl.c
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@ -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,