btrfs_free_extra_devids() is called only in the mount context which
traverses through the fs_devices::devices and frees the orphan devices
devices in the given %fs_devices if any. As the search for the orphan
device is limited to fs_devices::devices so we don't need the global
uuid_mutex.
There can't be any mount-point based ioctl threads in this context as
the mount thread is not yet returned. But there can be the btrfs-control
based scan ioctls thread which calls device_list_add().
Here in the mount thread the fs_devices::opened is incremented way before
btrfs_free_extra_devids() is called and in the scan context the fs_devices
which are already opened neither be freed or alloc-able at
device_list_add().
But lets say you change the device-path and call the scan again, then scan
would update the new device path and this operation could race against the
btrfs_free_extra_devids() thread, which might be in the process of
free-ing the same device. So synchronize it by using the
device_list_mutex.
This scenario is a very corner case, and practically the scan and mount
are anyway serialized by the usage so unless the race is instrumented its
very difficult to achieve.
Signed-off-by: Anand Jain <anand.j...@oracle.com>
---
Currently device_list_add() is very lean on its device_list_mutex usage,
a cleanup and fix is wip. Given the practicality of the above race
condition this patch is good to merge.
fs/btrfs/volumes.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index b6757b53c297..a9c1f4f7ebd0 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -925,7 +925,7 @@ void btrfs_free_extra_devids(struct btrfs_fs_devices
*fs_devices, int step)
struct btrfs_device *device, *next;
struct btrfs_device *latest_dev = NULL;
- mutex_lock(&uuid_mutex);
+ mutex_lock(&fs_devices->device_list_mutex);
again:
/* This is the initialized path, it is safe to release the devices. */
list_for_each_entry_safe(device, next, &fs_devices->devices, dev_list) {
@@ -979,8 +979,7 @@ void btrfs_free_extra_devids(struct btrfs_fs_devices
*fs_devices, int step)
}
fs_devices->latest_bdev = latest_dev->bdev;
-
- mutex_unlock(&uuid_mutex);
+ mutex_unlock(&fs_devices->device_list_mutex);
}
static void free_device_rcu(struct rcu_head *head)
--
2.7.0
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
