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porting: document new block device opening order

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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: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Christian Brauner 2023-09-15 16:01:02 +02:00
parent ae81711c1e
commit 2ba0dd6562
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Documentation/filesystems/porting.rst
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@ -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.