nfsd: optimise recalculate_deny_mode() for a common case

recalculate_deny_mode() takes time that is linear in the number of
stateids active on the file.

When called from
  release_openowner -> free_ol_stateid_reaplist ->nfs4_free_ol_stateid
  -> release_all_access

the number of times it is called is linear in the number of stateids.
The net result is that time taken by release_openowner is quadratic in
the number of stateids.

When the nfsd server is shut down while there are many active stateids
this can result in a soft lockup. ("CPU stuck for 302s" seen in one case).

In many cases all the states have the same deny modes and there is no
need to examine the entire list in recalculate_deny_mode().  In
particular, recalculate_deny_mode() will only reduce the deny mode,
never increase it.  So if some prefix of the list causes the original
deny mode to be required, there is no need to examine the remainder of
the list.

So we can improve recalculate_deny_mode() to usually run in constant
time, so release_openowner will typically be only linear in the number
of states.
Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4state.c

@@ -1397,11 +1397,16 @@ static void
 recalculate_deny_mode(struct nfs4_file *fp)
 {
 	struct nfs4_ol_stateid *stp;
+	u32 old_deny;
 
 	spin_lock(&fp->fi_lock);
+	old_deny = fp->fi_share_deny;
 	fp->fi_share_deny = 0;
-	list_for_each_entry(stp, &fp->fi_stateids, st_perfile)
+	list_for_each_entry(stp, &fp->fi_stateids, st_perfile) {
 		fp->fi_share_deny |= bmap_to_share_mode(stp->st_deny_bmap);
+		if (fp->fi_share_deny == old_deny)
+			break;
+	}
 	spin_unlock(&fp->fi_lock);
 }