[PATCH 04/17] btrfs: allocate scrub workqueues outside of locks

Josef Bacik
2020-08-10T15:42:58Z

I got the following lockdep splat while testing

======================================================
WARNING: possible circular locking dependency detected
5.8.0-rc7-00172-g021118712e59 #932 Not tainted
------------------------------------------------------
btrfs/229626 is trying to acquire lock:
ffffffff828513f0 (cpu_hotplug_lock){++++}-{0:0}, at: alloc_workqueue+0x378/0x450

but task is already holding lock:
ffff889dd3889518 (&fs_info->scrub_lock){+.+.}-{3:3}, at: btrfs_scrub_dev+0x11c/0x630

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #7 (&fs_info->scrub_lock){+.+.}-{3:3}:
__mutex_lock+0x9f/0x930
btrfs_scrub_dev+0x11c/0x630
btrfs_dev_replace_by_ioctl.cold.21+0x10a/0x1d4
btrfs_ioctl+0x2799/0x30a0
ksys_ioctl+0x83/0xc0
__x64_sys_ioctl+0x16/0x20
do_syscall_64+0x50/0x90
entry_SYSCALL_64_after_hwframe+0x44/0xa9

-> #6 (&fs_devs->device_list_mutex){+.+.}-{3:3}:
__mutex_lock+0x9f/0x930
btrfs_run_dev_stats+0x49/0x480
commit_cowonly_roots+0xb5/0x2a0
btrfs_commit_transaction+0x516/0xa60
sync_filesystem+0x6b/0x90
generic_shutdown_super+0x22/0x100
kill_anon_super+0xe/0x30
btrfs_kill_super+0x12/0x20
deactivate_locked_super+0x29/0x60
cleanup_mnt+0xb8/0x140
task_work_run+0x6d/0xb0
__prepare_exit_to_usermode+0x1cc/0x1e0
do_syscall_64+0x5c/0x90
entry_SYSCALL_64_after_hwframe+0x44/0xa9

-> #5 (&fs_info->tree_log_mutex){+.+.}-{3:3}:
__mutex_lock+0x9f/0x930
btrfs_commit_transaction+0x4bb/0xa60
sync_filesystem+0x6b/0x90
generic_shutdown_super+0x22/0x100
kill_anon_super+0xe/0x30
btrfs_kill_super+0x12/0x20
deactivate_locked_super+0x29/0x60
cleanup_mnt+0xb8/0x140
task_work_run+0x6d/0xb0
__prepare_exit_to_usermode+0x1cc/0x1e0
do_syscall_64+0x5c/0x90
entry_SYSCALL_64_after_hwframe+0x44/0xa9

-> #4 (&fs_info->reloc_mutex){+.+.}-{3:3}:
__mutex_lock+0x9f/0x930
btrfs_record_root_in_trans+0x43/0x70
start_transaction+0xd1/0x5d0
btrfs_dirty_inode+0x42/0xd0
touch_atime+0xa1/0xd0
btrfs_file_mmap+0x3f/0x60
mmap_region+0x3a4/0x640
do_mmap+0x376/0x580
vm_mmap_pgoff+0xd5/0x120
ksys_mmap_pgoff+0x193/0x230
do_syscall_64+0x50/0x90
entry_SYSCALL_64_after_hwframe+0x44/0xa9

-> #3 (&mm->mmap_lock#2){++++}-{3:3}:
__might_fault+0x68/0x90
_copy_to_user+0x1e/0x80
perf_read+0x141/0x2c0
vfs_read+0xad/0x1b0
ksys_read+0x5f/0xe0
do_syscall_64+0x50/0x90
entry_SYSCALL_64_after_hwframe+0x44/0xa9

-> #2 (&cpuctx_mutex){+.+.}-{3:3}:
__mutex_lock+0x9f/0x930
perf_event_init_cpu+0x88/0x150
perf_event_init+0x1db/0x20b
start_kernel+0x3ae/0x53c
secondary_startup_64+0xa4/0xb0

-> #1 (pmus_lock){+.+.}-{3:3}:
__mutex_lock+0x9f/0x930
perf_event_init_cpu+0x4f/0x150
cpuhp_invoke_callback+0xb1/0x900
_cpu_up.constprop.26+0x9f/0x130
cpu_up+0x7b/0xc0
bringup_nonboot_cpus+0x4f/0x60
smp_init+0x26/0x71
kernel_init_freeable+0x110/0x258
kernel_init+0xa/0x103
ret_from_fork+0x1f/0x30

-> #0 (cpu_hotplug_lock){++++}-{0:0}:
__lock_acquire+0x1272/0x2310
lock_acquire+0x9e/0x360
cpus_read_lock+0x39/0xb0
alloc_workqueue+0x378/0x450
__btrfs_alloc_workqueue+0x15d/0x200
btrfs_alloc_workqueue+0x51/0x160
scrub_workers_get+0x5a/0x170
btrfs_scrub_dev+0x18c/0x630
btrfs_dev_replace_by_ioctl.cold.21+0x10a/0x1d4
btrfs_ioctl+0x2799/0x30a0
ksys_ioctl+0x83/0xc0
__x64_sys_ioctl+0x16/0x20
do_syscall_64+0x50/0x90
entry_SYSCALL_64_after_hwframe+0x44/0xa9

other info that might help us debug this:

Chain exists of:
cpu_hotplug_lock --> &fs_devs->device_list_mutex --> &fs_info->scrub_lock

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&fs_info->scrub_lock);
lock(&fs_devs->device_list_mutex);
lock(&fs_info->scrub_lock);
lock(cpu_hotplug_lock);

*** DEADLOCK ***

2 locks held by btrfs/229626:
#0: ffff88bfe8bb86e0 (&fs_devs->device_list_mutex){+.+.}-{3:3}, at: btrfs_scrub_dev+0xbd/0x630
#1: ffff889dd3889518 (&fs_info->scrub_lock){+.+.}-{3:3}, at: btrfs_scrub_dev+0x11c/0x630

stack backtrace:
CPU: 15 PID: 229626 Comm: btrfs Kdump: loaded Not tainted 5.8.0-rc7-00172-g021118712e59 #932
Hardware name: Quanta Tioga Pass Single Side 01-0030993006/Tioga Pass Single Side, BIOS F08_3A18 12/20/2018
Call Trace:
dump_stack+0x78/0xa0
check_noncircular+0x165/0x180
__lock_acquire+0x1272/0x2310
lock_acquire+0x9e/0x360
? alloc_workqueue+0x378/0x450
cpus_read_lock+0x39/0xb0
? alloc_workqueue+0x378/0x450
alloc_workqueue+0x378/0x450
? rcu_read_lock_sched_held+0x52/0x80
__btrfs_alloc_workqueue+0x15d/0x200
btrfs_alloc_workqueue+0x51/0x160
scrub_workers_get+0x5a/0x170
btrfs_scrub_dev+0x18c/0x630
? start_transaction+0xd1/0x5d0
btrfs_dev_replace_by_ioctl.cold.21+0x10a/0x1d4
btrfs_ioctl+0x2799/0x30a0
? do_sigaction+0x102/0x250
? lockdep_hardirqs_on_prepare+0xca/0x160
? _raw_spin_unlock_irq+0x24/0x30
? trace_hardirqs_on+0x1c/0xe0
? _raw_spin_unlock_irq+0x24/0x30
? do_sigaction+0x102/0x250
? ksys_ioctl+0x83/0xc0
ksys_ioctl+0x83/0xc0
__x64_sys_ioctl+0x16/0x20
do_syscall_64+0x50/0x90
entry_SYSCALL_64_after_hwframe+0x44/0xa9

This happens because we're allocating the scrub workqueues under the
scrub and device list mutex, which brings in a whole host of other
dependencies. Fix this by moving the actual allocation outside of the
scrub lock, and then only take the lock once we're ready to actually
assign them to the fs_info. We'll now have to cleanup the workqueues in
a few more places, so I've added a helper to do the refcount dance to
safely free the workqueues.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
fs/btrfs/scrub.c | 122 +++++++++++++++++++++++++++--------------------
1 file changed, 70 insertions(+), 52 deletions(-)

diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index 5a6cb9db512e..7f7098e3110e 100644
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -3716,50 +3716,84 @@ static noinline_for_stack int scrub_supers(struct scrub_ctx *sctx,
return 0;
}

+static void scrub_workers_put(struct btrfs_fs_info *fs_info)
+{
+ if (refcount_dec_and_mutex_lock(&fs_info->scrub_workers_refcnt,
+ &fs_info->scrub_lock)) {
+ struct btrfs_workqueue *scrub_workers = NULL;
+ struct btrfs_workqueue *scrub_wr_comp = NULL;
+ struct btrfs_workqueue *scrub_parity = NULL;
+
+ scrub_workers = fs_info->scrub_workers;
+ scrub_wr_comp = fs_info->scrub_wr_completion_workers;
+ scrub_parity = fs_info->scrub_parity_workers;
+
+ fs_info->scrub_workers = NULL;
+ fs_info->scrub_wr_completion_workers = NULL;
+ fs_info->scrub_parity_workers = NULL;
+ mutex_unlock(&fs_info->scrub_lock);
+
+ btrfs_destroy_workqueue(scrub_workers);
+ btrfs_destroy_workqueue(scrub_wr_comp);
+ btrfs_destroy_workqueue(scrub_parity);
+ }
+}
+
/*
* get a reference count on fs_info->scrub_workers. start worker if necessary
*/
static noinline_for_stack int scrub_workers_get(struct btrfs_fs_info *fs_info,
int is_dev_replace)
{
+ struct btrfs_workqueue *scrub_workers = NULL;
+ struct btrfs_workqueue *scrub_wr_comp = NULL;
+ struct btrfs_workqueue *scrub_parity = NULL;
unsigned int flags = WQ_FREEZABLE | WQ_UNBOUND;
int max_active = fs_info->thread_pool_size;
+ int ret = -ENOMEM;

- lockdep_assert_held(&fs_info->scrub_lock);
+ if (refcount_inc_not_zero(&fs_info->scrub_workers_refcnt))
+ return 0;

- if (refcount_read(&fs_info->scrub_workers_refcnt) == 0) {
- ASSERT(fs_info->scrub_workers == NULL);
- fs_info->scrub_workers = btrfs_alloc_workqueue(fs_info, "scrub",
- flags, is_dev_replace ? 1 : max_active, 4);
- if (!fs_info->scrub_workers)
- goto fail_scrub_workers;
-
- ASSERT(fs_info->scrub_wr_completion_workers == NULL);
- fs_info->scrub_wr_completion_workers =
- btrfs_alloc_workqueue(fs_info, "scrubwrc", flags,
- max_active, 2);
- if (!fs_info->scrub_wr_completion_workers)
- goto fail_scrub_wr_completion_workers;
+ scrub_workers = btrfs_alloc_workqueue(fs_info, "scrub", flags,
+ is_dev_replace ? 1 : max_active,
+ 4);
+ if (!scrub_workers)
+ goto fail_scrub_workers;

- ASSERT(fs_info->scrub_parity_workers == NULL);
- fs_info->scrub_parity_workers =
- btrfs_alloc_workqueue(fs_info, "scrubparity", flags,
+ scrub_wr_comp = btrfs_alloc_workqueue(fs_info, "scrubwrc", flags,
max_active, 2);
- if (!fs_info->scrub_parity_workers)
- goto fail_scrub_parity_workers;
+ if (!scrub_wr_comp)
+ goto fail_scrub_wr_completion_workers;

+ scrub_parity = btrfs_alloc_workqueue(fs_info, "scrubparity", flags,
+ max_active, 2);
+ if (!scrub_parity)
+ goto fail_scrub_parity_workers;
+
+ mutex_lock(&fs_info->scrub_lock);
+ if (refcount_read(&fs_info->scrub_workers_refcnt) == 0) {
+ ASSERT(fs_info->scrub_workers == NULL &&
+ fs_info->scrub_wr_completion_workers == NULL &&
+ fs_info->scrub_parity_workers == NULL);
+ fs_info->scrub_workers = scrub_workers;
+ fs_info->scrub_wr_completion_workers = scrub_wr_comp;
+ fs_info->scrub_parity_workers = scrub_parity;
refcount_set(&fs_info->scrub_workers_refcnt, 1);
- } else {
- refcount_inc(&fs_info->scrub_workers_refcnt);
+ mutex_unlock(&fs_info->scrub_lock);
+ return 0;
}
- return 0;
+ refcount_inc(&fs_info->scrub_workers_refcnt);
+ mutex_unlock(&fs_info->scrub_lock);

+ ret = 0;
+ btrfs_destroy_workqueue(scrub_parity);
fail_scrub_parity_workers:
- btrfs_destroy_workqueue(fs_info->scrub_wr_completion_workers);
+ btrfs_destroy_workqueue(scrub_wr_comp);
fail_scrub_wr_completion_workers:
- btrfs_destroy_workqueue(fs_info->scrub_workers);
+ btrfs_destroy_workqueue(scrub_workers);
fail_scrub_workers:
- return -ENOMEM;
+ return ret;
}

int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
@@ -3770,9 +3804,6 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
int ret;
struct btrfs_device *dev;
unsigned int nofs_flag;
- struct btrfs_workqueue *scrub_workers = NULL;
- struct btrfs_workqueue *scrub_wr_comp = NULL;
- struct btrfs_workqueue *scrub_parity = NULL;

if (btrfs_fs_closing(fs_info))
return -EAGAIN;
@@ -3819,13 +3850,17 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
if (IS_ERR(sctx))
return PTR_ERR(sctx);

+ ret = scrub_workers_get(fs_info, is_dev_replace);
+ if (ret)
+ goto out_free_ctx;
+
mutex_lock(&fs_info->fs_devices->device_list_mutex);
dev = btrfs_find_device(fs_info->fs_devices, devid, NULL, NULL, true);
if (!dev || (test_bit(BTRFS_DEV_STATE_MISSING, &dev->dev_state) &&
!is_dev_replace)) {
mutex_unlock(&fs_info->fs_devices->device_list_mutex);
ret = -ENODEV;
- goto out_free_ctx;
+ goto out;
}

if (!is_dev_replace && !readonly &&
@@ -3834,7 +3869,7 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
btrfs_err_in_rcu(fs_info, "scrub: device %s is not writable",
rcu_str_deref(dev->name));
ret = -EROFS;
- goto out_free_ctx;
+ goto out;
}

mutex_lock(&fs_info->scrub_lock);
@@ -3843,7 +3878,7 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
mutex_unlock(&fs_info->scrub_lock);
mutex_unlock(&fs_info->fs_devices->device_list_mutex);
ret = -EIO;
- goto out_free_ctx;
+ goto out;
}

down_read(&fs_info->dev_replace.rwsem);
@@ -3854,17 +3889,10 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
mutex_unlock(&fs_info->scrub_lock);
mutex_unlock(&fs_info->fs_devices->device_list_mutex);
ret = -EINPROGRESS;
- goto out_free_ctx;
+ goto out;
}
up_read(&fs_info->dev_replace.rwsem);

- ret = scrub_workers_get(fs_info, is_dev_replace);
- if (ret) {
- mutex_unlock(&fs_info->scrub_lock);
- mutex_unlock(&fs_info->fs_devices->device_list_mutex);
- goto out_free_ctx;
- }
-
sctx->readonly = readonly;
dev->scrub_ctx = sctx;
mutex_unlock(&fs_info->fs_devices->device_list_mutex);
@@ -3917,24 +3945,14 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,

mutex_lock(&fs_info->scrub_lock);
dev->scrub_ctx = NULL;
- if (refcount_dec_and_test(&fs_info->scrub_workers_refcnt)) {
- scrub_workers = fs_info->scrub_workers;
- scrub_wr_comp = fs_info->scrub_wr_completion_workers;
- scrub_parity = fs_info->scrub_parity_workers;
-
- fs_info->scrub_workers = NULL;
- fs_info->scrub_wr_completion_workers = NULL;
- fs_info->scrub_parity_workers = NULL;
- }
mutex_unlock(&fs_info->scrub_lock);

- btrfs_destroy_workqueue(scrub_workers);
- btrfs_destroy_workqueue(scrub_wr_comp);
- btrfs_destroy_workqueue(scrub_parity);
+ scrub_workers_put(fs_info);
scrub_put_ctx(sctx);

return ret;
-
+out:
+ scrub_workers_put(fs_info);
out_free_ctx:
scrub_free_ctx(sctx);

--
2.24.1