super: retire sget(), convert iterators to RCU [LWN.net]

From:		Christian Brauner <brauner-AT-kernel.org>
To:		linux-fsdevel-AT-vger.kernel.org
Subject:		[PATCH 0/8] super: retire sget(), convert iterators to RCU
Date:		Tue, 26 May 2026 17:09:02 +0200
Message-ID:		<20260526-work-sget-v1-0-263f7025cedd@kernel.org>
Cc:		Theodore Ts'o <tytso-AT-mit.edu>, Andreas Dilger <adilger.kernel-AT-dilger.ca>, Jan Kara <jack-AT-suse.cz>, "Ritesh Harjani (IBM)" <ritesh.list-AT-gmail.com>, linux-ext4-AT-vger.kernel.org, linux-cifs-AT-vger.kernel.org, Alexander Viro <viro-AT-zeniv.linux.org.uk>, "Christian Brauner (Amutable)" <brauner-AT-kernel.org>
Archive-link:		Article

* retire sget(): CIFS plus the two ext4 KUnit tests (extents-test,
  mballoc-test) were the last in-tree callers, and all three convert
  cleanly to sget_fc(). That lets sget() and its prototype come out,
  taking ~60 lines that only existed to be kept in lockstep with
  sget_fc() on every publish-path change.

* Walk @super_blocks and @type->fs_supers under RCU, pinned by
  refcount_inc_not_zero(&sb->s_count). iterate_supers(),
  iterate_supers_type(), user_get_super(), do_emergency_remount(),
  filesystems_freeze() and filesystems_thaw() no longer hold sb_lock
  across the cursor advance.

  The conversion goes in four small steps. Drop sb_lock from
  setup_bdev_super(): the {s_bdev_file, s_bdev, s_bdi,
  SB_I_STABLE_WRITES} tuple is publication of immutable state, and
  SB_BORN already gates every reader via super_wake()'s
  smp_store_release paired with super_flags()'s smp_load_acquire. Then
  convert sb->s_count to refcount_t -- mechanical, every increment is
  still under sb_lock. Then switch the write-side list/hlist ops to
  their _rcu variants; @super_blocks gets list_bidir_del_rcu() so the
  reverse-walking iterators (filesystems_freeze, do_emergency_remount)
  keep a valid ->prev on the unlinked entry, matching the canonical
  pattern in kernel/nstree.c. Finally, convert the iterators themselves:
  cursor advance via READ_ONCE / rcu_dereference, with the previous
  entry kept pinned via its s_count across the rcu_read_unlock ->
  callback -> rcu_read_lock cycle.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
Christian Brauner (8):
      ext4: convert extents KUnit test to sget_fc()
      ext4: convert mballoc KUnit test to sget_fc()
      smb: client: convert cifs_smb3_do_mount() to sget_fc()
      fs: retire sget()
      super: drop sb_lock from setup_bdev_super() tuple publication
      super: convert sb->s_count to refcount_t
      super: switch list manipulation to _rcu primitives
      super: convert iterators to RCU readers + refcount_inc_not_zero

 fs/btrfs/super.c               |   2 +-
 fs/ext4/extents-test.c         |  22 +++++-
 fs/ext4/mballoc-test.c         |  17 ++++-
 fs/smb/client/cifsfs.c         |  43 ++++++-----
 fs/smb/client/cifsfs.h         |   3 +-
 fs/smb/client/cifsproto.h      |   3 +-
 fs/smb/client/connect.c        |   5 +-
 fs/smb/client/fs_context.c     |   2 +-
 fs/super.c                     | 167 ++++++++++++++---------------------------
 include/linux/fs.h             |   4 -
 include/linux/fs/super_types.h |   3 +-
 11 files changed, 127 insertions(+), 144 deletions(-)
---
base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731
change-id: 20260526-work-sget-6bc80b96cba5