Merge: NFSv4/pNFS: Fix a race to wake on NFS_LAYOUT_DRAIN

MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-9/-/merge_requests/7105

JIRA: https://issues.redhat.com/browse/RHEL-92556

commit c017762
Author: Benjamin Coddington <bcodding@redhat.com>
Date:   Thu Jun 19 11:02:21 2025 -0400

    NFSv4/pNFS: Fix a race to wake on NFS_LAYOUT_DRAIN

    We found a few different systems hung up in writeback waiting on the same
    page lock, and one task waiting on the NFS_LAYOUT_DRAIN bit in
    pnfs_update_layout(), however the pnfs_layout_hdr's plh_outstanding count
    was zero.

    It seems most likely that this is another race between the waiter and waker
    similar to commit ed0172a ("SUNRPC: Fix a race to wake a sync task").
    Fix it up by applying the advised barrier.

    Fixes: 880265c ("pNFS: Avoid a live lock condition in pnfs_update_layout()")
    Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
    Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>

Approved-by: Scott Mayhew <smayhew@redhat.com>
Approved-by: Olga Kornievskaia <okorniev@redhat.com>
Approved-by: CKI KWF Bot <cki-ci-bot+kwf-gitlab-com@redhat.com>

Merged-by: Augusto Caringi <acaringi@redhat.com>

Author: caringi

fs/nfs/pnfs.c

@@ -2042,8 +2042,10 @@ static void nfs_layoutget_begin(struct pnfs_layout_hdr *lo)
 static void nfs_layoutget_end(struct pnfs_layout_hdr *lo)
 {
 	if (atomic_dec_and_test(&lo->plh_outstanding) &&
-	    test_and_clear_bit(NFS_LAYOUT_DRAIN, &lo->plh_flags))
+	    test_and_clear_bit(NFS_LAYOUT_DRAIN, &lo->plh_flags)) {
+		smp_mb__after_atomic();
 		wake_up_bit(&lo->plh_flags, NFS_LAYOUT_DRAIN);
+	}
 }
 
 static bool pnfs_is_first_layoutget(struct pnfs_layout_hdr *lo)