[PATCH] Btrfs: avoid deadlock with memory reclaim due to allocation of devices

[fdmanana@kernel.org]

From: Filipe Manana <fdmanana@suse.com>

Several places allocate a device while holding the device list mutex. This
can result in a deadlock if reclaim happens because the device, and its
flush bio, are allocated using GFP_KERNEL mode (by __alloc_device() which
is used by btrfs_alloc_device()). A transaction commit, which reclaim can
trigger, needs to lock the device list mutex in its critical section, done
at btrfs_update_commit_device_size().

Some of these places are device_list_add(), which ends up being called
through the device scan ioctl, and btrfs_close_one_device(), which ends up
being called through the device remove ioctl.

Since all the places that add elements to the list of resized devices (the
device grow and shrink functions) only lock the chunk mutex before adding
a device to the list, drop the need to acquire the device list mutex from
btrfs_update_commit_device_size(), which is the only other place that uses
this list and it already locks the chunk mutex.

Fixes: 78f2c9e6dbb14 ("btrfs: device add and remove: use GFP_KERNEL")
Fixes: e0ae999414238 ("btrfs: preallocate device flush bio")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/volumes.c | 8 ++------
 fs/btrfs/volumes.h | 1 +
 2 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index c872adfc939e..74c4ed29e36e 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -176,7 +176,8 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info,
  * chunk_mutex
  * -----------
  * protects chunks, adding or removing during allocation, trim or when a new
- * device is added/removed
+ * device is added/removed, and the list of resized devices at struct
+ * btrfs_fs_info::fs_devices::resized_devices
  *
  * cleaner_mutex
  * -------------
@@ -7298,10 +7299,6 @@ void btrfs_update_commit_device_size(struct btrfs_fs_info *fs_info)
 	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
 	struct btrfs_device *curr, *next;
 
-	if (list_empty(&fs_devices->resized_devices))
-		return;
-
-	mutex_lock(&fs_devices->device_list_mutex);
 	mutex_lock(&fs_info->chunk_mutex);
 	list_for_each_entry_safe(curr, next, &fs_devices->resized_devices,
 				 resized_list) {
@@ -7309,7 +7306,6 @@ void btrfs_update_commit_device_size(struct btrfs_fs_info *fs_info)
 		curr->commit_total_bytes = curr->disk_total_bytes;
 	}
 	mutex_unlock(&fs_info->chunk_mutex);
-	mutex_unlock(&fs_devices->device_list_mutex);
 }
 
 /* Must be invoked during the transaction commit */
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index aefce895e994..362574b9c37a 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -229,6 +229,7 @@ struct btrfs_fs_devices {
 	struct mutex device_list_mutex;
 	struct list_head devices;
 
+	/* protected by struct btrfs_fs_info::chunk_mutex */
 	struct list_head resized_devices;
 	/* devices not currently being allocated */
 	struct list_head alloc_list;
-- 
2.11.0