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.jain@xxxxxxxxxx>
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
---
v3->v4: As we traverse through the seed device, fs_device gets updated with
the child seed fs_devices, so make sure we use the same fs_devices
pointer for the mutex_unlock as used for the mutex_lock.
v2->v3: Update change log.
(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).
v1->v2: replace uuid_mutex with device_list_mutex instead of delete.
change log updated.
fs/btrfs/volumes.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 88d37bfa99c8..870c9f69a6a4 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -934,8 +934,9 @@ void btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices, int step)
{
struct btrfs_device *device, *next;
struct btrfs_device *latest_dev = NULL;
+ struct btrfs_fs_devices *parent_fs_devices = fs_devices;
- mutex_lock(&uuid_mutex);
+ mutex_lock(&parent_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) {
@@ -989,8 +990,7 @@ 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(&parent_fs_devices->device_list_mutex);
}
static void free_device_rcu(struct rcu_head *head)
--
2.7.0
--
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