[PATCH 1/3] NFSv4: Fix a livelock when CLOSE pre-emptively bumps state sequence

[Benjamin Coddington <bcodding@redhat.com>]

Since commit 0e0cb35b417f ("NFSv4: Handle NFS4ERR_OLD_STATEID in
CLOSE/OPEN_DOWNGRADE") the following livelock may occur if a CLOSE races
with the update of the nfs_state:

Process 1	  Process 2	   Server
=========         =========	   ========
 OPEN file
		  OPEN file
		  		   Reply OPEN (1)
		  		   Reply OPEN (2)
 Update state (1)
 CLOSE file (1)
		  		   Reply OLD_STATEID (1)
 CLOSE file (2)
		  		   Reply CLOSE (-1)
		  Update state (2)
		  wait for state change
 OPEN file
		  wake
 CLOSE file
 OPEN file
		  wake
 CLOSE file
 ...
		  ...

As long as the first process continues updating state, the second process
will fail to exit the loop in nfs_set_open_stateid_locked().  This livelock
has been observed in generic/168.

Fix this by detecting the case in nfs_need_update_open_stateid() and
then exit the loop if:
 - the state is NFS_OPEN_STATE, and
 - the stateid sequence is > 1, and
 - the stateid doesn't match the current open stateid

Fixes: 0e0cb35b417f ("NFSv4: Handle NFS4ERR_OLD_STATEID in CLOSE/OPEN_DOWNGRADE")
Cc: stable@vger.kernel.org # v5.4+
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
---
 fs/nfs/nfs4proc.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index 45e0585e0667..9ced7a62c05e 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -1570,10 +1570,14 @@ static bool nfs_need_update_open_stateid(struct nfs4_state *state,
 {
 	if (test_bit(NFS_OPEN_STATE, &state->flags) == 0 ||
 	    !nfs4_stateid_match_other(stateid, &state->open_stateid)) {
-		if (stateid->seqid == cpu_to_be32(1))
+		if (stateid->seqid == cpu_to_be32(1)) {
 			nfs_state_log_update_open_stateid(state);
-		else
-			set_bit(NFS_STATE_CHANGE_WAIT, &state->flags);
+		} else {
+			if (!nfs4_stateid_match_other(stateid, &state->open_stateid))
+				return false;
+			else
+				set_bit(NFS_STATE_CHANGE_WAIT, &state->flags);
+		}
 		return true;
 	}
 
-- 
2.20.1