do_reconfigure_mnt() used to take the down_write(&sb->s_umount) lock
which seems unnecessary since we're not changing the superblock. We're
only checking whether it is already read-only. Setting other mount
attributes is protected by lock_mount_hash() afaict and not by s_umount.
The history of down_write(&sb->s_umount) lock being taken when setting
mount attributes dates back to the introduction of MNT_READONLY in [2].
This introduced the concept of having read-only mounts in contrast to
just having a read-only superblock. When it got introduced it was simply
plumbed into do_remount() which already took down_write(&sb->s_umount)
because it was only used to actually change the superblock before [2].
Afaict, it would've already been possible back then to only use
down_read(&sb->s_umount) for MS_BIND | MS_REMOUNT since actual mount
options were protected by the vfsmount lock already. But that would've
meant special casing the locking for MS_BIND | MS_REMOUNT in
do_remount() which people might not have considered worth it.
Then in [1] MS_BIND | MS_REMOUNT mount option changes were split out of
do_remount() into do_reconfigure_mnt() but the down_write(&sb->s_umount)
lock was simply copied over.
Now that we have this be a separate helper only take the
down_read(&sb->s_umount) lock since we're only interested in checking
whether the super block is currently read-only and blocking any writers
from changing it. Essentially, checking that the super block is
read-only has the advantage that we can avoid having to go into the
slowpath and through MNT_WRITE_HOLD and can simply set the read-only
flag on the mount in set_mount_attributes().
[1]: commit 43f5e655eff7 ("vfs: Separate changing mount flags full remount")
[2]: commit 2e4b7fcd9260 ("[PATCH] r/o bind mounts: honor mount writer counts at remount")
Link: https://lore.kernel.org/r/[email protected]
Cc: David Howells <[email protected]>
Cc: Al Viro <[email protected]>
Cc: [email protected]
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>
---
/* v2 */
unchanged
/* v3 */
unchanged
/* v4 */
unchanged
/* v5 */
unchanged
base-commit: 7c53f6b671f4aba70ff15e1b05148b10d58c2837
/* v6 */
unchanged
base-commit: 19c329f6808995b142b3966301f217c831e7cf31
---
fs/namespace.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
diff --git a/fs/namespace.c b/fs/namespace.c
index 5fceb2854395..367f1c7cb6db 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2527,10 +2527,6 @@ static int change_mount_ro_state(struct mount *mnt, unsigned int mnt_flags)
return 0;
}
-/*
- * Update the user-settable attributes on a mount. The caller must hold
- * sb->s_umount for writing.
- */
static void set_mount_attributes(struct mount *mnt, unsigned int mnt_flags)
{
mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;
@@ -2580,13 +2576,17 @@ static int do_reconfigure_mnt(struct path *path, unsigned int mnt_flags)
if (!can_change_locked_flags(mnt, mnt_flags))
return -EPERM;
- down_write(&sb->s_umount);
+ /*
+ * We're only checking whether the superblock is read-only not
+ * changing it, so only take down_read(&sb->s_umount).
+ */
+ down_read(&sb->s_umount);
lock_mount_hash();
ret = change_mount_ro_state(mnt, mnt_flags);
if (ret == 0)
set_mount_attributes(mnt, mnt_flags);
unlock_mount_hash();
- up_write(&sb->s_umount);
+ up_read(&sb->s_umount);
mnt_warn_timestamp_expiry(path, &mnt->mnt);
--
2.30.0