[PATCH RFC] NFSD: Fix possible sleep during nfsd4_release_lockowner()

[Chuck Lever <chuck.lever@oracle.com>]

nfsd4_release_lockowner() holds clp->cl_lock when it calls
check_for_locks(). However, check_for_locks() calls nfsd_file_get()
/ nfsd_file_put() to access the backing inode's flc_posix list, and
nfsd_file_put() can sleep if the inode was recently removed.

Let's instead rely on the stateowner's reference count to gate
whether the release is permitted. This should be a reliable
indication of locks-in-use since file lock operations and
->lm_get_owner take appropriate references, which are released
appropriately when file locks are removed.

Reported-by: Dai Ngo <dai.ngo@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: stable@vger.kernel.org
---
 fs/nfsd/nfs4state.c |    9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

This might be a naive approach, but let's start with it.

This passes light testing, but it's not clear how much our existing
fleet of tests exercises this area. I've locally built a couple of
pynfs tests (one is based on the one Dai posted last week) and they
pass too.

I don't believe that FREE_STATEID needs the same simplification.

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index a280256cbb03..b77894e668a4 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -7559,12 +7559,9 @@ nfsd4_release_lockowner(struct svc_rqst *rqstp,
 
 		/* see if there are still any locks associated with it */
 		lo = lockowner(sop);
-		list_for_each_entry(stp, &sop->so_stateids, st_perstateowner) {
-			if (check_for_locks(stp->st_stid.sc_file, lo)) {
-				status = nfserr_locks_held;
-				spin_unlock(&clp->cl_lock);
-				return status;
-			}
+		if (atomic_read(&sop->so_count) > 1) {
+			spin_unlock(&clp->cl_lock);
+			return nfserr_locks_held;
 		}
 
 		nfs4_get_stateowner(sop);