[PATCH 03/10] nfsd: serialize nfsd4_end_grace() with atomic test-and-set

[Jeff Layton <jlayton@kernel.org>]

From: Chris Mason <clm@meta.com>

nfsd4_end_grace() guards its drain path with a plain bool:

    if (nn->grace_ended)
            return;
    nn->grace_ended = true;

The read and the write are independent, and nothing in struct
nfsd_net serializes them.  At least two contexts can reach this
code with no lock held:

    laundromat path
      laundry_wq kworker
        nfs4_laundromat()
          nfsd4_end_grace()

    RECLAIM_COMPLETE path
      nfsd compound kthread
        nfsd4_reclaim_complete()
          inc_reclaim_complete()
            nfsd4_end_grace()

Both callers can observe grace_ended == false on different CPUs,
both store true, and both proceed into nfsd4_record_grace_done(),
which invokes the active client_tracking_ops->grace_done callback.
For tracking ops that drain reclaim_str_hashtbl (legacy_tracking_ops
via nfsd4_recdir_purge_old, and the cld v1+ ops via
nfsd4_cld_grace_done), grace_done calls nfs4_release_reclaim(),
which walks every bucket of reclaim_str_hashtbl with no lock and
calls nfs4_remove_reclaim_record() (list_del + kfree) on each
entry.  Two concurrent walkers corrupt the list and double-free
every nfs4_client_reclaim.  A concurrent nfsd4_find_reclaim_client()
iterating the same bucket reads through freed memory.

A third call site exists in nfs4_state_start_net() on the
skip_grace startup path, but it runs under nfsd_mutex before any
client has connected and before the laundromat's first delayed
work fires, so it cannot race with the two callers above.

Fix by replacing the read/write pair with try_cmpxchg() so exactly
one caller transitions grace_ended from false to true and proceeds
into the drain; the loser returns immediately.  bool supports
1-byte cmpxchg on all supported architectures, and no lock
ordering changes are needed.

Fixes: 362063a595be ("nfsd: keep a tally of RECLAIM_COMPLETE operations when using nfsdcld")
Assisted-by: kres:claude-opus-4-7
Signed-off-by: Chris Mason <clm@meta.com>
---
 fs/nfsd/nfs4state.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index f4d12dbcf97b..dc4ac541436f 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -7022,12 +7022,23 @@ nfsd4_renew(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 static void
 nfsd4_end_grace(struct nfsd_net *nn)
 {
-	/* do nothing if grace period already ended */
-	if (nn->grace_ended)
+	bool expected = false;
+
+	/*
+	 * nfsd4_end_grace() can be entered concurrently from the
+	 * laundromat workqueue and from an nfsd compound thread
+	 * handling RECLAIM_COMPLETE.  Without serialization, both
+	 * callers can observe grace_ended==false and proceed into
+	 * nfsd4_record_grace_done().  For tracking ops whose
+	 * grace_done drains reclaim_str_hashtbl, that results in
+	 * list corruption and a double free of every
+	 * nfs4_client_reclaim entry.  Use an atomic test-and-set so
+	 * exactly one caller proceeds.
+	 */
+	if (!try_cmpxchg(&nn->grace_ended, &expected, true))
 		return;
 
 	trace_nfsd_grace_complete(nn);
-	nn->grace_ended = true;
 	/*
 	 * If the server goes down again right now, an NFSv4
 	 * client will still be allowed to reclaim after it comes back up,

-- 
2.54.0