nfsd: replace rp_mutex to avoid deadlock in move_to_close_lru()

move_to_close_lru() waits for sc_count to become zero while holding
rp_mutex.  This can deadlock if another thread holds a reference and is
waiting for rp_mutex.

By the time we get to move_to_close_lru() the openowner is unhashed and
cannot be found any more.  So code waiting for the mutex can safely
retry the lookup if move_to_close_lru() has started.

So change rp_mutex to an atomic_t with three states:

 RP_UNLOCK   - state is still hashed, not locked for reply
 RP_LOCKED   - state is still hashed, is locked for reply
 RP_UNHASHED - state is not hashed, no code can get a lock.

Use wait_var_event() to wait for either a lock, or for the owner to be
unhashed.  In the latter case, retry the lookup.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
index 2ed0fcf879fd..a6261754deed 100644
--- a/fs/nfsd/state.h
+++ b/fs/nfsd/state.h
@@ -486,7 +486,7 @@ struct nfs4_replay {
 	unsigned int		rp_buflen;
 	char			*rp_buf;
 	struct knfsd_fh		rp_openfh;
-	struct mutex		rp_mutex;
+	atomic_t		rp_locked;
 	char			rp_ibuf[NFSD4_REPLAY_ISIZE];
 };