Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx> --- Documentation/filesystems/porting.rst | 25 +++++++++++++++++++++++++ 1 file changed, 25 insertions(+) diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst index 4d05b9862451..fef97a2e6729 100644 --- a/Documentation/filesystems/porting.rst +++ b/Documentation/filesystems/porting.rst @@ -1045,3 +1045,28 @@ filesystem type is now moved to a later point when the devices are closed: As this is a VFS level change it has no practical consequences for filesystems other than that all of them must use one of the provided kill_litter_super(), kill_anon_super(), or kill_block_super() helpers. + +--- + +**mandatory** + +Block device freezing and thawing have been moved to holder operations. As we +can now go straight from block devcie to superblock the get_active_super() +and bd_fsfreeze_sb members in struct block_device are gone. + +The bd_fsfreeze_mutex is gone as well since we can rely on the bd_holder_lock +to protect against concurrent freeze and thaw. + +Before this change, get_active_super() would only be able to find the +superblock of the main block device, i.e., the one stored in sb->s_bdev. Block +device freezing now works for any block device owned by a given superblock, not +just the main block device. + +When thawing we now grab an active reference so we can hold bd_holder_lock +across thaw without the risk of deadlocks (because the superblock goes away +which would require us to take bd_holder_lock). That allows us to get rid of +bd_fsfreeze_mutex. Currently we just reacquire s_umount after thaw_super() and +drop the active reference we took before. This someone could grab an active +reference before we dropped the last one. This shouldn't be an issue. If it +turns out to be one we can reshuffle the code to simply hold s_umount when +thaw_super() returns and drop the reference. -- 2.34.1
