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author | Jiri Kosina <jkosina@suse.cz> | 2023-11-01 02:07:35 +0300 |
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committer | Jiri Kosina <jkosina@suse.cz> | 2023-11-01 02:07:35 +0300 |
commit | 20cd569d7ee8fce24e8753f0f43af6c420557b1f (patch) | |
tree | f559cfda594846795aa51c99d96f92d8c912851a /Documentation/filesystems/porting.rst | |
parent | 62cc9c3cb3ec1bf31cc116146185ed97b450836a (diff) | |
parent | eeebfe6259ba2d5b0980eb7b0df384eb77e9e4f5 (diff) | |
download | linux-20cd569d7ee8fce24e8753f0f43af6c420557b1f.tar.xz |
Merge branch 'for-6.7/config_pm' into for-linus
- #ifdef CONFIG_PM removal from HID code (Thomas Weißschuh)
Diffstat (limited to 'Documentation/filesystems/porting.rst')
-rw-r--r-- | Documentation/filesystems/porting.rst | 96 |
1 files changed, 96 insertions, 0 deletions
diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst index deac4e973ddc..4d05b9862451 100644 --- a/Documentation/filesystems/porting.rst +++ b/Documentation/filesystems/porting.rst @@ -949,3 +949,99 @@ 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. + +--- + +**mandatory** + +The holder of a block device is now the superblock. + +The holder of a block device used to be the file_system_type which wasn't +particularly useful. It wasn't possible to go from block device to owning +superblock without matching on the device pointer stored in the superblock. +This mechanism would only work for a single device so the block layer couldn't +find the owning superblock of any additional devices. + +In the old mechanism reusing or creating a superblock for a racing mount(2) and +umount(2) relied on the file_system_type as the holder. This was severly +underdocumented however: + +(1) Any concurrent mounter that managed to grab an active reference on an + existing superblock was made to wait until the superblock either became + ready or until the superblock was removed from the list of superblocks of + the filesystem type. If the superblock is ready the caller would simple + reuse it. + +(2) If the mounter came after deactivate_locked_super() but before + the superblock had been removed from the list of superblocks of the + filesystem type the mounter would wait until the superblock was shutdown, + reuse the block device and allocate a new superblock. + +(3) If the mounter came after deactivate_locked_super() and after + the superblock had been removed from the list of superblocks of the + filesystem type the mounter would reuse the block device and allocate a new + superblock (the bd_holder point may still be set to the filesystem type). + +Because the holder of the block device was the file_system_type any concurrent +mounter could open the block devices of any superblock of the same +file_system_type without risking seeing EBUSY because the block device was +still in use by another superblock. + +Making the superblock the owner of the block device changes this as the holder +is now a unique superblock and thus block devices associated with it cannot be +reused by concurrent mounters. So a concurrent mounter in (2) could suddenly +see EBUSY when trying to open a block device whose holder was a different +superblock. + +The new logic thus waits until the superblock and the devices are shutdown in +->kill_sb(). Removal of the superblock from the list of superblocks of the +filesystem type is now moved to a later point when the devices are closed: + +(1) Any concurrent mounter managing to grab an active reference on an existing + superblock is made to wait until the superblock is either ready or until + the superblock and all devices are shutdown in ->kill_sb(). If the + superblock is ready the caller will simply reuse it. + +(2) If the mounter comes after deactivate_locked_super() but before + the superblock has been removed from the list of superblocks of the + filesystem type the mounter is made to wait until the superblock and the + devices are shut down in ->kill_sb() and the superblock is removed from the + list of superblocks of the filesystem type. The mounter will allocate a new + superblock and grab ownership of the block device (the bd_holder pointer of + the block device will be set to the newly allocated superblock). + +(3) This case is now collapsed into (2) as the superblock is left on the list + of superblocks of the filesystem type until all devices are shutdown in + ->kill_sb(). In other words, if the superblock isn't on the list of + superblock of the filesystem type anymore then it has given up ownership of + all associated block devices (the bd_holder pointer is NULL). + +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. |