Patch "CIFS: fix oplock break deadlocks" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

CIFS: fix oplock break deadlocks

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
cifs-fix-oplock-break-deadlocks.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Rabin Vincent <[email protected]>
Date: Wed, 3 May 2017 17:54:01 +0200
Subject: CIFS: fix oplock break deadlocks

From: Rabin Vincent <[email protected]>

commit 3998e6b87d4258a70df358296d6f1c7234012bfe upstream.

When the final cifsFileInfo_put() is called from cifsiod and an oplock break work is queued, lockdep complains loudly:

=============================================
[ INFO: possible recursive locking detected ]
4.11.0+ #21 Not tainted
---------------------------------------------
kworker/0:2/78 is trying to acquire lock:
("cifsiod"){++++.+}, at: flush_work+0x215/0x350

but task is already holding lock:
("cifsiod"){++++.+}, at: process_one_work+0x255/0x8e0

other info that might help us debug this:

  Possible unsafe locking scenario:

        CPU0
        ----

lock("cifsiod");
lock("cifsiod");

*** DEADLOCK ***

May be due to missing lock nesting notation

2 locks held by kworker/0:2/78:
#0: ("cifsiod"){++++.+}, at: process_one_work+0x255/0x8e0 #1: ((&wdata->work)){+.+...}, at: process_one_work+0x255/0x8e0

 stack backtrace:
 CPU: 0 PID: 78 Comm: kworker/0:2 Not tainted 4.11.0+ #21
 Workqueue: cifsiod cifs_writev_complete
 Call Trace:
  dump_stack+0x85/0xc2
  __lock_acquire+0x17dd/0x2260
  ? match_held_lock+0x20/0x2b0
  ? trace_hardirqs_off_caller+0x86/0x130
  ? mark_lock+0xa6/0x920
  lock_acquire+0xcc/0x260
  ? lock_acquire+0xcc/0x260
  ? flush_work+0x215/0x350
  flush_work+0x236/0x350
  ? flush_work+0x215/0x350
  ? destroy_worker+0x170/0x170
  __cancel_work_timer+0x17d/0x210
  ? ___preempt_schedule+0x16/0x18
  cancel_work_sync+0x10/0x20
  cifsFileInfo_put+0x338/0x7f0
  cifs_writedata_release+0x2a/0x40
  ? cifs_writedata_release+0x2a/0x40
  cifs_writev_complete+0x29d/0x850
  ? preempt_count_sub+0x18/0xd0
  process_one_work+0x304/0x8e0
  worker_thread+0x9b/0x6a0
  kthread+0x1b2/0x200
  ? process_one_work+0x8e0/0x8e0
  ? kthread_create_on_node+0x40/0x40
  ret_from_fork+0x31/0x40

This is a real warning. Since the oplock is queued on the same workqueue this can deadlock if there is only one worker thread active for the workqueue (which will be the case during memory pressure when the rescuer thread is handling it).

Furthermore, there is at least one other kind of hang possible due to the oplock break handling if there is only worker. (This can be reproduced without introducing memory pressure by having passing 1 for the max_active parameter of cifsiod.) cifs_oplock_break() can wait indefintely in the filemap_fdatawait() while the cifs_writev_complete() work is blocked:

sysrq: SysRq : Show Blocked State
task PC stack pid father
kworker/0:1 D 0 16 2 0x00000000

 Workqueue: cifsiod cifs_oplock_break
 Call Trace:
  __schedule+0x562/0xf40
  ? mark_held_locks+0x4a/0xb0
  schedule+0x57/0xe0
  io_schedule+0x21/0x50
  wait_on_page_bit+0x143/0x190
  ? add_to_page_cache_lru+0x150/0x150
  __filemap_fdatawait_range+0x134/0x190
  ? do_writepages+0x51/0x70
  filemap_fdatawait_range+0x14/0x30
  filemap_fdatawait+0x3b/0x40
  cifs_oplock_break+0x651/0x710
  ? preempt_count_sub+0x18/0xd0
  process_one_work+0x304/0x8e0
  worker_thread+0x9b/0x6a0
  kthread+0x1b2/0x200
  ? process_one_work+0x8e0/0x8e0
  ? kthread_create_on_node+0x40/0x40
  ret_from_fork+0x31/0x40
 dd              D    0   683    171 0x00000000
 Call Trace:
  __schedule+0x562/0xf40
  ? mark_held_locks+0x29/0xb0
  schedule+0x57/0xe0
  io_schedule+0x21/0x50
  wait_on_page_bit+0x143/0x190
  ? add_to_page_cache_lru+0x150/0x150
  __filemap_fdatawait_range+0x134/0x190
  ? do_writepages+0x51/0x70
  filemap_fdatawait_range+0x14/0x30
  filemap_fdatawait+0x3b/0x40
  filemap_write_and_wait+0x4e/0x70
  cifs_flush+0x6a/0xb0
  filp_close+0x52/0xa0
  __close_fd+0xdc/0x150
  SyS_close+0x33/0x60
  entry_SYSCALL_64_fastpath+0x1f/0xbe

 Showing all locks held in the system:
 2 locks held by kworker/0:1/16:
  #0:  ("cifsiod"){.+.+.+}, at: process_one_work+0x255/0x8e0
  #1:  ((&cfile->oplock_break)){+.+.+.}, at: process_one_work+0x255/0x8e0

Showing busy workqueues and worker pools:
workqueue cifsiod: flags=0xc
pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/1
in-flight: 16:cifs_oplock_break
delayed: cifs_writev_complete, cifs_echo_request
pool 0: cpus=0 node=0 flags=0x0 nice=0 hung=0s workers=3 idle: 750 3

Fix these problems by creating a a new workqueue (with a rescuer) for the oplock break work.

---
 fs/cifs/cifsfs.c   |   15 +++++++++++++--
 fs/cifs/cifsglob.h |    1 +
 fs/cifs/misc.c     |    2 +-
 fs/cifs/smb2misc.c |    5 +++--
 4 files changed, 18 insertions(+), 5 deletions(-)

@@ -87,6 +87,7 @@ extern mempool_t *cifs_req_poolp;
 extern mempool_t *cifs_mid_poolp;
 
 struct workqueue_struct	*cifsiod_wq;
+struct workqueue_struct	*cifsoplockd_wq;
 __u32 cifs_lock_secret;
 
 /*

@@ -1283,9 +1284,16 @@ init_cifs(void)
 		goto out_clean_proc;
 	}
 
+	cifsoplockd_wq = alloc_workqueue("cifsoplockd",
+					 WQ_FREEZABLE|WQ_MEM_RECLAIM, 0);
+	if (!cifsoplockd_wq) {
+		rc = -ENOMEM;
+		goto out_destroy_cifsiod_wq;
+	}
+
 	rc = cifs_fscache_register();
 	if (rc)
-		goto out_destroy_wq;
+		goto out_destroy_cifsoplockd_wq;
 
 	rc = cifs_init_inodecache();
 	if (rc)

@@ -1333,7 +1341,9 @@ out_destroy_inodecache:
 	cifs_destroy_inodecache();
 out_unreg_fscache:
 	cifs_fscache_unregister();
-out_destroy_wq:
+out_destroy_cifsoplockd_wq:
+	destroy_workqueue(cifsoplockd_wq);
+out_destroy_cifsiod_wq:
 	destroy_workqueue(cifsiod_wq);
 out_clean_proc:
 	cifs_proc_clean();

@@ -1356,6 +1366,7 @@ exit_cifs(void)
 	cifs_destroy_mids();
 	cifs_destroy_inodecache();
 	cifs_fscache_unregister();
+	destroy_workqueue(cifsoplockd_wq);
 	destroy_workqueue(cifsiod_wq);
 	cifs_proc_clean();
 }

@@ -1651,6 +1651,7 @@ void cifs_oplock_break(struct work_struc
 
 extern const struct slow_work_ops cifs_oplock_break_ops;
 extern struct workqueue_struct *cifsiod_wq;
+extern struct workqueue_struct *cifsoplockd_wq;
 extern __u32 cifs_lock_secret;
 
 extern mempool_t *cifs_mid_poolp;

@@ -492,7 +492,7 @@ is_valid_oplock_break(char *buffer, stru
 					   CIFS_INODE_DOWNGRADE_OPLOCK_TO_L2,
 					   &pCifsInode->flags);
 
-				queue_work(cifsiod_wq,
+				queue_work(cifsoplockd_wq,
 					   &netfile->oplock_break);
 				netfile->oplock_break_cancelled = false;
 

@@ -494,7 +494,7 @@ smb2_tcon_has_lease(struct cifs_tcon *tc
 		else
 			cfile->oplock_break_cancelled = true;
 
-		queue_work(cifsiod_wq, &cfile->oplock_break);
+		queue_work(cifsoplockd_wq, &cfile->oplock_break);
 		kfree(lw);
 		return true;
 	}

@@ -638,7 +638,8 @@ smb2_is_valid_oplock_break(char *buffer,
 					   CIFS_INODE_DOWNGRADE_OPLOCK_TO_L2,
 					   &cinode->flags);
 				spin_unlock(&cfile->file_info_lock);
-				queue_work(cifsiod_wq, &cfile->oplock_break);
+				queue_work(cifsoplockd_wq,
+					   &cfile->oplock_break);
 
 				spin_unlock(&tcon->open_file_lock);
 				spin_unlock(&cifs_tcp_ses_lock);

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/cifs-fix-oplock-break-deadlocks.patch