btrfs: allocate scrub workqueues outside of locks

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.

Because the work queue allocation is done with GFP_KERNEL, it can
trigger reclaim, which can lead to a transaction commit, which in turns
needs the device_list_mutex, it can lead to a deadlock. A different
problem for which this fix is a solution.

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.

CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

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;
+	/* Other thread raced in and created the workers for us */
+	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);