[PATCH 2/2] NFSv4: Fix dropped lock for racing OPEN and delegation return

[Benjamin Coddington <bcodding@redhat.com>]

Commmit f5ea16137a3f ("NFSv4: Retry LOCK on OLD_STATEID during delegation
return") attempted to solve this problem by using nfs4's generic async error
handling, but introduced a regression where v4.0 lock recovery would hang.
The additional complexity introduced by overloading that error handling is
not necessary for this case.

The problem as originally explained in the above commit is:

    There's a small window where a LOCK sent during a delegation return can
    race with another OPEN on client, but the open stateid has not yet been
    updated.  In this case, the client doesn't handle the OLD_STATEID error
    from the server and will lose this lock, emitting:
    "NFS: nfs4_handle_delegation_recall_error: unhandled error -10024".

We want a fix that is much more focused to the original problem.  Fix this
issue by returning -EAGAIN from the nfs4_handle_delegation_recall_error() on
OLD_STATEID, and use that as a signal for the delegation return code to
retry the LOCK operation.  We should at this point be able to send along
the updated stateid.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
---
 fs/nfs/delegation.c | 4 +++-
 fs/nfs/nfs4proc.c   | 1 +
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/nfs/delegation.c b/fs/nfs/delegation.c
index cf7365581031..23aeb02319a5 100644
--- a/fs/nfs/delegation.c
+++ b/fs/nfs/delegation.c
@@ -160,7 +160,9 @@ static int nfs_delegation_claim_locks(struct nfs4_state *state, const nfs4_state
 		if (nfs_file_open_context(fl->fl_file)->state != state)
 			continue;
 		spin_unlock(&flctx->flc_lock);
-		status = nfs4_lock_delegation_recall(fl, state, stateid);
+		do {
+			status = nfs4_lock_delegation_recall(fl, state, stateid);
+		} while (status == -EAGAIN);
 		if (status < 0)
 			goto out;
 		spin_lock(&flctx->flc_lock);
diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index 6bb14f6cfbc0..399db73a57f4 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -2262,6 +2262,7 @@ static int nfs4_handle_delegation_recall_error(struct nfs_server *server, struct
 		case -NFS4ERR_BAD_HIGH_SLOT:
 		case -NFS4ERR_CONN_NOT_BOUND_TO_SESSION:
 		case -NFS4ERR_DEADSESSION:
+		case -NFS4ERR_OLD_STATEID:
 			return -EAGAIN;
 		case -NFS4ERR_STALE_CLIENTID:
 		case -NFS4ERR_STALE_STATEID:
-- 
2.40.1