Unverified Commit 2ba0dd65 authored by Christian Brauner's avatar Christian Brauner
Browse files

porting: document new block device opening order 



We've changed the order of opening block devices and superblock
handling. Let's document this so filesystem and vfs developers have
a proper digital paper trail.

Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent ae81711c

Documentation/filesystems/porting.rst

+26 −0
Original line number Diff line number Diff line
@@ -949,3 +949,29 @@ mmap_lock held. All in-tree users have been audited and do not seem to 
depend on the mmap_lock being held, but out of tree users should verify

for themselves.  If they do need it, they can return VM_FAULT_RETRY to

be called with the mmap_lock held.



---



**mandatory**



The order of opening block devices and matching or creating superblocks has

changed.



The old logic opened block devices first and then tried to find a

suitable superblock to reuse based on the block device pointer.



The new logic tries to find a suitable superblock first based on the device

number, and opening the block device afterwards.



Since opening block devices cannot happen under s_umount because of lock

ordering requirements s_umount is now dropped while opening block devices and

reacquired before calling fill_super().



In the old logic concurrent mounters would find the superblock on the list of

superblocks for the filesystem type. Since the first opener of the block device

would hold s_umount they would wait until the superblock became either born or

was discarded due to initialization failure.



Since the new logic drops s_umount concurrent mounters could grab s_umount and

would spin. Instead they are now made to wait using an explicit wait-wake

mechanism without having to hold s_umount.