[PATCH 5/8] super: drop sb_lock from setup_bdev_super() tuple publication

[Christian Brauner <brauner@kernel.org>]

The tuple {s_bdev_file, s_bdev, s_bdi, SB_I_STABLE_WRITES} written by
setup_bdev_super() is publication of immutable state, not list
integrity. The sb is already on @super_blocks and @fs_supers at this
point (sget_dev() -> sget_fc() put it there) but SB_BORN is unset, so
any iterator that calls super_lock() blocks on
wait_var_event(SB_BORN | SB_DYING).

The SUPER_ITER_UNLOCKED iterators (filesystems_freeze,
filesystems_thaw, do_emergency_remount) do not look at s_bdev, s_bdi
or s_iflags so they cannot observe a partial fill either.

When vfs_get_tree() later calls super_wake(sb, SB_BORN) it does

    smp_store_release(&sb->s_flags, sb->s_flags | SB_BORN)

and any reader gating on SB_BORN via super_flags() loads sb->s_flags
with smp_load_acquire(). The release/acquire pair orders the four
prior writes against the load of SB_BORN.

s_iflags is a shared field so use WRITE_ONCE() on the
read-modify-write to keep the compiler from tearing the store.
retire_super() is the only other writer of s_iflags and only runs
against an already-born sb under s_umount.

This drops one of the five sb_lock acquisitions in the mount path
with no behavioural change for any reader.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/super.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 5fe8cea9f8fe..c451f689c7b3 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1576,13 +1576,16 @@ int setup_bdev_super(struct super_block *sb, int sb_flags,
 		bdev_fput(bdev_file);
 		return -EBUSY;
 	}
-	spin_lock(&sb_lock);
+	/*
+	 * Publish before SB_BORN is set. super_wake(sb, SB_BORN) below uses
+	 * smp_store_release(); any iterator that observes SB_BORN via
+	 * super_flags()'s smp_load_acquire() sees these writes.
+	 */
 	sb->s_bdev_file = bdev_file;
 	sb->s_bdev = bdev;
 	sb->s_bdi = bdi_get(bdev->bd_disk->bdi);
 	if (bdev_stable_writes(bdev))
-		sb->s_iflags |= SB_I_STABLE_WRITES;
-	spin_unlock(&sb_lock);
+		WRITE_ONCE(sb->s_iflags, sb->s_iflags | SB_I_STABLE_WRITES);
 
 	snprintf(sb->s_id, sizeof(sb->s_id), "%pg", bdev);
 	shrinker_debugfs_rename(sb->s_shrink, "sb-%s:%s", sb->s_type->name,

-- 
2.47.3