[PATCH v2 2/3] btrfs: scrub: fix circular locking dependency warning

[Anand Jain <anand.jain@oracle.com>]

Circular locking dependency check reports warning[1], that's because
the btrfs_scrub_dev() calls the stack #0 below with, the
fs_info::scrub_lock held. The test case leading to this warning..

  mkfs.btrfs -fq /dev/sdb && mount /dev/sdb /btrfs
  btrfs scrub start -B /btrfs

In fact we have fs_info::scrub_workers_refcnt to tack if the init and
destroy of the scrub workers are needed. So once we have incremented
and decremented the fs_info::scrub_workers_refcnt value in the thread,
its ok to drop the scrub_lock, and then actually do the
btrfs_destroy_workqueue() part. So this patch drops the scrub_lock
before calling btrfs_destroy_workqueue().

[1]
[   76.146826] ======================================================
[   76.147086] WARNING: possible circular locking dependency detected
[   76.147316] 4.20.0-rc3+ #41 Not tainted
[   76.147489] ------------------------------------------------------
[   76.147722] btrfs/4065 is trying to acquire lock:
[   76.147984] 0000000038593bc0 ((wq_completion)"%s-%s""btrfs",
name){+.+.}, at: flush_workqueue+0x70/0x4d0
[   76.148337]
but task is already holding lock:
[   76.148594] 0000000062392ab7 (&fs_info->scrub_lock){+.+.}, at:
btrfs_scrub_dev+0x316/0x5d0 [btrfs]
[   76.148909]
which lock already depends on the new lock.

[   76.149191]
the existing dependency chain (in reverse order) is:
[   76.149446]
-> #3 (&fs_info->scrub_lock){+.+.}:
[   76.149707]        btrfs_scrub_dev+0x11f/0x5d0 [btrfs]
[   76.149924]        btrfs_ioctl+0x1ac3/0x2d80 [btrfs]
[   76.150216]        do_vfs_ioctl+0xa9/0x6d0
[   76.150468]        ksys_ioctl+0x60/0x90
[   76.150716]        __x64_sys_ioctl+0x16/0x20
[   76.150911]        do_syscall_64+0x50/0x180
[   76.151182]        entry_SYSCALL_64_after_hwframe+0x49/0xbe
[   76.151469]
-> #2 (&fs_devs->device_list_mutex){+.+.}:
[   76.151851]        reada_start_machine_worker+0xca/0x3f0 [btrfs]
[   76.152195]        normal_work_helper+0xf0/0x4c0 [btrfs]
[   76.152489]        process_one_work+0x1f4/0x520
[   76.152751]        worker_thread+0x46/0x3d0
[   76.153715]        kthread+0xf8/0x130
[   76.153912]        ret_from_fork+0x3a/0x50
[   76.154178]
-> #1 ((work_completion)(&work->normal_work)){+.+.}:
[   76.154575]        worker_thread+0x46/0x3d0
[   76.154828]        kthread+0xf8/0x130
[   76.155108]        ret_from_fork+0x3a/0x50
[   76.155357]
-> #0 ((wq_completion)"%s-%s""btrfs", name){+.+.}:
[   76.155751]        flush_workqueue+0x9a/0x4d0
[   76.155911]        drain_workqueue+0xca/0x1a0
[   76.156182]        destroy_workqueue+0x17/0x230
[   76.156455]        btrfs_destroy_workqueue+0x5d/0x1c0 [btrfs]
[   76.156756]        scrub_workers_put+0x2e/0x60 [btrfs]
[   76.156931]        btrfs_scrub_dev+0x329/0x5d0 [btrfs]
[   76.157219]        btrfs_ioctl+0x1ac3/0x2d80 [btrfs]
[   76.157491]        do_vfs_ioctl+0xa9/0x6d0
[   76.157742]        ksys_ioctl+0x60/0x90
[   76.157910]        __x64_sys_ioctl+0x16/0x20
[   76.158177]        do_syscall_64+0x50/0x180
[   76.158429]        entry_SYSCALL_64_after_hwframe+0x49/0xbe
[   76.158716]
other info that might help us debug this:

[   76.158908] Chain exists of:
  (wq_completion)"%s-%s""btrfs", name --> &fs_devs->device_list_mutex
--> &fs_info->scrub_lock

[   76.159629]  Possible unsafe locking scenario:

[   76.160607]        CPU0                    CPU1
[   76.160934]        ----                    ----
[   76.161210]   lock(&fs_info->scrub_lock);
[   76.161458]
lock(&fs_devs->device_list_mutex);
[   76.161805]
lock(&fs_info->scrub_lock);
[   76.161909]   lock((wq_completion)"%s-%s""btrfs", name);
[   76.162201]
 *** DEADLOCK ***

[   76.162627] 2 locks held by btrfs/4065:
[   76.162897]  #0: 00000000bef2775b (sb_writers#12){.+.+}, at:
mnt_want_write_file+0x24/0x50
[   76.163335]  #1: 0000000062392ab7 (&fs_info->scrub_lock){+.+.}, at:
btrfs_scrub_dev+0x316/0x5d0 [btrfs]
[   76.163796]
stack backtrace:
[   76.163911] CPU: 1 PID: 4065 Comm: btrfs Not tainted 4.20.0-rc3+ #41
[   76.164228] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS
VirtualBox 12/01/2006
[   76.164646] Call Trace:
[   76.164872]  dump_stack+0x5e/0x8b
[   76.165128]  print_circular_bug.isra.37+0x1f1/0x1fe
[   76.165398]  __lock_acquire+0x14aa/0x1620
[   76.165652]  lock_acquire+0xb0/0x190
[   76.165910]  ? flush_workqueue+0x70/0x4d0
[   76.166175]  flush_workqueue+0x9a/0x4d0
[   76.166420]  ? flush_workqueue+0x70/0x4d0
[   76.166671]  ? drain_workqueue+0x52/0x1a0
[   76.166911]  drain_workqueue+0xca/0x1a0
[   76.167167]  destroy_workqueue+0x17/0x230
[   76.167428]  btrfs_destroy_workqueue+0x5d/0x1c0 [btrfs]
[   76.167720]  scrub_workers_put+0x2e/0x60 [btrfs]
[   76.168233]  btrfs_scrub_dev+0x329/0x5d0 [btrfs]
[   76.168504]  ? __sb_start_write+0x121/0x1b0
[   76.168759]  ? mnt_want_write_file+0x24/0x50
[   76.169654]  btrfs_ioctl+0x1ac3/0x2d80 [btrfs]
[   76.169934]  ? find_held_lock+0x2d/0x90
[   76.170204]  ? find_held_lock+0x2d/0x90
[   76.170450]  do_vfs_ioctl+0xa9/0x6d0
[   76.170690]  ? __fget+0x101/0x1f0
[   76.170910]  ? __fget+0x5/0x1f0
[   76.171157]  ksys_ioctl+0x60/0x90
[   76.171391]  __x64_sys_ioctl+0x16/0x20
[   76.171634]  do_syscall_64+0x50/0x180
[   76.171892]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[   76.172186] RIP: 0033:0x7f61d422e567
[   76.172425] Code: 44 00 00 48 8b 05 29 09 2d 00 64 c7 00 26 00 00 00
48 c7 c0 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 b8 10 00 00 00 0f
05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d f9 08 2d 00 f7 d8 64 89 01 48
[   76.172911] RSP: 002b:00007f61d3936d68 EFLAGS: 00000246 ORIG_RAX:
0000000000000010
[   76.173328] RAX: ffffffffffffffda RBX: 00000000019026b0 RCX:
00007f61d422e567
[   76.173649] RDX: 00000000019026b0 RSI: 00000000c400941b RDI:
0000000000000003
[   76.173909] RBP: 0000000000000000 R08: 00007f61d3937700 R09:
0000000000000000
[   76.174244] R10: 00007f61d3937700 R11: 0000000000000246 R12:
0000000000000000
[   76.174566] R13: 0000000000801000 R14: 0000000000000000 R15:
00007f61d3937700
[   76.175217] btrfs (4065) used greatest stack depth: 11424 bytes left

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
v1->v2: none
 fs/btrfs/scrub.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index b5a19ba38ab7..9ade0659f017 100644
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -3757,10 +3757,13 @@ static noinline_for_stack int scrub_workers_get(struct btrfs_fs_info *fs_info,
 
 static noinline_for_stack void scrub_workers_put(struct btrfs_fs_info *fs_info)
 {
+	lockdep_assert_held(&fs_info->scrub_lock);
 	if (--fs_info->scrub_workers_refcnt == 0) {
+		mutex_unlock(&fs_info->scrub_lock);
 		btrfs_destroy_workqueue(fs_info->scrub_workers);
 		btrfs_destroy_workqueue(fs_info->scrub_wr_completion_workers);
 		btrfs_destroy_workqueue(fs_info->scrub_parity_workers);
+		mutex_lock(&fs_info->scrub_lock);
 	}
 	WARN_ON(fs_info->scrub_workers_refcnt < 0);
 }
-- 
1.8.3.1