[PATCH v4 01/40] namespace: take lock_mount_hash() directly when changing flags

[Christian Brauner]

Changing mount options always ends up taking lock_mount_hash() but when
MNT_READONLY is requested and neither the mount nor the superblock are
MNT_READONLY we end up taking the lock, dropping it, and retaking it to
change the other mount attributes. Instead, let's acquire the lock once
when changing the mount attributes. This simplifies the locking in these
codepath, makes them easier to reason about and avoids having to
reacquire the lock right after dropping it.

Cc: David Howells <dhowells at redhat.com>
Cc: Al Viro <viro at zeniv.linux.org.uk>
Cc: linux-fsdevel at vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>
Reviewed-by: Christoph Hellwig <hch at lst.de>
---
/* v2 */
- Christoph Hellwig <hch at lst.de>:
  - Remove pointless __mnt_unmake_readonly() helper.
  - Even though Christoph suggested to lockdep_assert_held() into places that
    require {lock,unlock}_mount_hash() it seems that seqlock's don't support
    it.

/* v3 */
unchanged

/* v4 */
unchanged
---
 fs/namespace.c | 22 ++++++++--------------
 1 file changed, 8 insertions(+), 14 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index cebaa3e81794..f183161833ad 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -463,7 +463,6 @@ static int mnt_make_readonly(struct mount *mnt)
 {
 	int ret = 0;
 
-	lock_mount_hash();
 	mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
 	/*
 	 * After storing MNT_WRITE_HOLD, we'll read the counters. This store
@@ -497,18 +496,9 @@ static int mnt_make_readonly(struct mount *mnt)
 	 */
 	smp_wmb();
 	mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
-	unlock_mount_hash();
 	return ret;
 }
 
-static int __mnt_unmake_readonly(struct mount *mnt)
-{
-	lock_mount_hash();
-	mnt->mnt.mnt_flags &= ~MNT_READONLY;
-	unlock_mount_hash();
-	return 0;
-}
-
 int sb_prepare_remount_readonly(struct super_block *sb)
 {
 	struct mount *mnt;
@@ -2508,7 +2498,8 @@ static int change_mount_ro_state(struct mount *mnt, unsigned int mnt_flags)
 	if (readonly_request)
 		return mnt_make_readonly(mnt);
 
-	return __mnt_unmake_readonly(mnt);
+	mnt->mnt.mnt_flags &= ~MNT_READONLY;
+	return 0;
 }
 
 /*
@@ -2517,11 +2508,9 @@ static int change_mount_ro_state(struct mount *mnt, unsigned int mnt_flags)
  */
 static void set_mount_attributes(struct mount *mnt, unsigned int mnt_flags)
 {
-	lock_mount_hash();
 	mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;
 	mnt->mnt.mnt_flags = mnt_flags;
 	touch_mnt_namespace(mnt->mnt_ns);
-	unlock_mount_hash();
 }
 
 static void mnt_warn_timestamp_expiry(struct path *mountpoint, struct vfsmount *mnt)
@@ -2567,9 +2556,11 @@ static int do_reconfigure_mnt(struct path *path, unsigned int mnt_flags)
 		return -EPERM;
 
 	down_write(&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);
 
 	mnt_warn_timestamp_expiry(path, &mnt->mnt);
@@ -2610,8 +2601,11 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
 		err = -EPERM;
 		if (ns_capable(sb->s_user_ns, CAP_SYS_ADMIN)) {
 			err = reconfigure_super(fc);
-			if (!err)
+			if (!err) {
+				lock_mount_hash();
 				set_mount_attributes(mnt, mnt_flags);
+				unlock_mount_hash();
+			}
 		}
 		up_write(&sb->s_umount);
 	}
-- 
2.29.2