Commit 1e414adf authored by Al Viro's avatar Al Viro
Browse files

WRITE_HOLD machinery: no need for to bump mount_lock seqcount 



... neither for insertion into the list of instances, nor for
mnt_{un,}hold_writers(), nor for mnt_get_write_access() deciding
to be nice to RT during a busy-wait loop - all of that only needs
the spinlock side of mount_lock.

IOW, it's mount_locked_reader, not mount_writer.

Clarify the comment re locking rules for mnt_unhold_writers() - it's
not just that mount_lock needs to be held when calling that, it must
have been held all along since the matching mnt_hold_writers().

Reviewed-by: default avatarChristian Brauner <brauner@kernel.org>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 3371fa2f

fs/namespace.c

+8 −8
Original line number Diff line number Diff line
@@ -526,8 +526,8 @@ int mnt_get_write_access(struct vfsmount *m) 
			 * the same CPU as the task that is spinning here.

			 */

			preempt_enable();

			lock_mount_hash();

			unlock_mount_hash();

			read_seqlock_excl(&mount_lock);

			read_sequnlock_excl(&mount_lock);

			preempt_disable();

		}

	}
@@ -671,7 +671,7 @@ EXPORT_SYMBOL(mnt_drop_write_file); 
 * a call to mnt_unhold_writers() in order to stop preventing write access to

 * @mnt.

 *

 * Context: This function expects lock_mount_hash() to be held serializing

 * Context: This function expects to be in mount_locked_reader scope serializing

 *          setting WRITE_HOLD.

 * Return: On success 0 is returned.

 *	   On error, -EBUSY is returned.
@@ -716,7 +716,8 @@ static inline int mnt_hold_writers(struct mount *mnt) 
 *

 * This function can only be called after a call to mnt_hold_writers().

 *

 * Context: This function expects lock_mount_hash() to be held.

 * Context: This function expects to be in the same mount_locked_reader scope

 * as the matching mnt_hold_writers().

 */

static inline void mnt_unhold_writers(struct mount *mnt)

{
@@ -770,7 +771,8 @@ int sb_prepare_remount_readonly(struct super_block *sb) 
	if (atomic_long_read(&sb->s_remove_count))

		return -EBUSY;



	lock_mount_hash();

	guard(mount_locked_reader)();



	for (struct mount *m = sb->s_mounts; m; m = m->mnt_next_for_sb) {

		if (!(m->mnt.mnt_flags & MNT_READONLY)) {

			err = mnt_hold_writers(m);
@@ -787,7 +789,6 @@ int sb_prepare_remount_readonly(struct super_block *sb) 
		if (test_write_hold(m))

			clear_write_hold(m);

	}

	unlock_mount_hash();



	return err;

}
@@ -1226,9 +1227,8 @@ static void setup_mnt(struct mount *m, struct dentry *root) 
	m->mnt_mountpoint = m->mnt.mnt_root;

	m->mnt_parent = m;



	lock_mount_hash();

	guard(mount_locked_reader)();

	mnt_add_instance(m, s);

	unlock_mount_hash();

}



/**