From acc58d0bab55a50e02c25f00bd6a210ee121595f Mon Sep 17 00:00:00 2001
From: Pavel Shilovsky <pshilov@microsoft.com>
Date: Thu, 17 Jan 2019 08:21:24 -0800
Subject: [PATCH] CIFS: Fix possible hang during async MTU reads and writes
Git-commit: acc58d0bab55a50e02c25f00bd6a210ee121595f
Patch-mainline: v5.0-rc4
References: bsc#1051510, bsc#1144333

When doing MTU i/o we need to leave some credits for
possible reopen requests and other operations happening
in parallel. Currently we leave 1 credit which is not
enough even for reopen only: we need at least 2 credits
if durable handle reconnect fails. Also there may be
other operations at the same time including compounding
ones which require 3 credits at a time each. Fix this
by leaving 8 credits which is big enough to cover most
scenarios.

Was able to reproduce this when server was configured
to give out fewer credits than usual.

The proper fix would be to reconnect a file handle first
and then obtain credits for an MTU request but this leads
to bigger code changes and should happen in other patches.

Cc: <stable@vger.kernel.org>
Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Acked-by: Paulo Alcantara <palcantara@suse.de>
---
 fs/cifs/smb2ops.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
index cf7eb891804f..1d3ce127b02e 100644
--- a/fs/cifs/smb2ops.c
+++ b/fs/cifs/smb2ops.c
@@ -165,14 +165,14 @@ smb2_wait_mtu_credits(struct TCP_Server_Info *server, unsigned int size,
 
 			scredits = server->credits;
 			/* can deadlock with reopen */
-			if (scredits == 1) {
+			if (scredits <= 8) {
 				*num = SMB2_MAX_BUFFER_SIZE;
 				*credits = 0;
 				break;
 			}
 
-			/* leave one credit for a possible reopen */
-			scredits--;
+			/* leave some credits for reopen and other ops */
+			scredits -= 8;
 			*num = min_t(unsigned int, size,
 				     scredits * SMB2_MAX_BUFFER_SIZE);
 
-- 
2.16.4