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author | Christian Brauner <christian.brauner@ubuntu.com> | 2021-01-21 14:19:50 +0100 |
---|---|---|
committer | Christian Brauner <christian.brauner@ubuntu.com> | 2021-01-24 14:29:34 +0100 |
commit | e58ace1a0fa9d578f85f556b4b88c5fe9b871d08 (patch) | |
tree | 4c38b6b80548f40e11c3bb03ba031d6b42bd8dac /fs | |
parent | d033cb6784c4f3a19a593cfe11f850e476197388 (diff) | |
download | linux-e58ace1a0fa9d578f85f556b4b88c5fe9b871d08.tar.bz2 |
namespace: only take read lock in do_reconfigure_mnt()
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/20210121131959.646623-32-christian.brauner@ubuntu.com
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Diffstat (limited to 'fs')
-rw-r--r-- | fs/namespace.c | 12 |
1 files 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); |