Linux-Fsdevel Archive on
help / color / mirror / Atom feed
From: Christian Brauner <>
To: David Howells <>,
	Al Viro <>,
	Michael Kerrisk <>,
	Christian Brauner <>
Subject: [PATCH 2/4] namespace: only take read lock in do_reconfigure_mnt()
Date: Tue, 14 Jul 2020 18:14:14 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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.

So I think 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]. Afaict, 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")
Cc: Al Viro <>
Cc: David Howells <>
Signed-off-by: Christian Brauner <>
 fs/namespace.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 395b9c912edf..ab025dd3be04 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2507,10 +2507,6 @@ static int change_mount_ro_state(struct mount *mnt, unsigned int mnt_flags)
 	return __mnt_unmake_readonly(mnt);
- * 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;
@@ -2560,13 +2556,17 @@ 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);
 	ret = change_mount_ro_state(mnt, mnt_flags);
 	if (ret == 0)
 		set_mount_attributes(mnt, mnt_flags);
-	up_write(&sb->s_umount);
+	up_read(&sb->s_umount);
 	mnt_warn_timestamp_expiry(path, &mnt->mnt);

  parent reply	other threads:[~2020-07-14 16:15 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-14 16:14 [PATCH 0/4] fs: add mount_setattr() Christian Brauner
2020-07-14 16:14 ` [PATCH] mount_setattr.2: New manual page documenting the mount_setattr() system call Christian Brauner
2020-07-21  9:59   ` Michael Kerrisk (man-pages)
2020-07-14 16:14 ` [PATCH 1/4] namespace: take lock_mount_hash() directly when changing flags Christian Brauner
2020-07-14 16:49   ` Jann Horn
2020-07-14 16:14 ` Christian Brauner [this message]
2020-07-14 16:14 ` [PATCH 3/4] fs: add mount_setattr() Christian Brauner
2020-07-15  8:29   ` Geert Uytterhoeven
2020-07-15 11:02   ` Christian Brauner
2020-07-14 16:14 ` [PATCH 4/4] tests: add mount_setattr() selftests Christian Brauner
2020-07-19 17:10 ` [PATCH 0/4] fs: add mount_setattr() Al Viro
2020-07-19 17:55   ` Christian Brauner
2020-10-01 14:17 ` Pavel Tikhomirov

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).