During remount of a bind mount (mount -o remount,bind,ro,... /mnt/mntpt), we currently take down_write(&sb->s_umount). This causes the remount operation to get blocked behind writes occuring on device (possibly mounted somewhere else). We have observed that simply trying to change the bind-mount from read-write to read-only can take several seconds becuase writeback is in progress (which takes read-lock on sb->s_umount). Since we are not modifying the superblock during MS_BIND remount, it should be enough to take a read-lock on sb->s_umount instead of the write-lock to protect against parallel unmounts and (in conjunction with vfsmount_lock) parallel remounts. This patch fixes the locking so that the MS_BIND remount changes happen only under down_read(&sb->s_umount).
Changes from RFC: Instead of completely removing the locking in MS_BIND case, take read-lock on sb->s_umount to protect against parallel unmount. Signed-off-by: Aditya Kali <[email protected]> --- fs/namespace.c | 42 ++++++++++++++++++++++++++++-------------- 1 file changed, 28 insertions(+), 14 deletions(-) diff --git a/fs/namespace.c b/fs/namespace.c index da5c494..cf74920 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -454,11 +454,13 @@ void mnt_drop_write_file(struct file *file) } EXPORT_SYMBOL(mnt_drop_write_file); +/* + * Must be called under br_write_lock(&vfsmount_lock). + */ static int mnt_make_readonly(struct mount *mnt) { int ret = 0; - br_write_lock(&vfsmount_lock); mnt->mnt.mnt_flags |= MNT_WRITE_HOLD; /* * After storing MNT_WRITE_HOLD, we'll read the counters. This store @@ -492,15 +494,15 @@ static int mnt_make_readonly(struct mount *mnt) */ smp_wmb(); mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD; - br_write_unlock(&vfsmount_lock); return ret; } +/* + * Must be called under br_write_lock(&vfsmount_lock). + */ static void __mnt_unmake_readonly(struct mount *mnt) { - br_write_lock(&vfsmount_lock); mnt->mnt.mnt_flags &= ~MNT_READONLY; - br_write_unlock(&vfsmount_lock); } int sb_prepare_remount_readonly(struct super_block *sb) @@ -1796,6 +1798,9 @@ out: return err; } +/* + * Must be called under br_write_lock(&vfsmount_lock). + */ static int change_mount_flags(struct vfsmount *mnt, int ms_flags) { int error = 0; @@ -1838,20 +1843,29 @@ static int do_remount(struct path *path, int flags, int mnt_flags, if (err) return err; - down_write(&sb->s_umount); - if (flags & MS_BIND) + if (flags & MS_BIND) { + down_read(&sb->s_umount); + br_write_lock(&vfsmount_lock); err = change_mount_flags(path->mnt, flags); - else if (!capable(CAP_SYS_ADMIN)) + if (!err) { + mnt_flags |= mnt->mnt.mnt_flags & MNT_PROPAGATION_MASK; + mnt->mnt.mnt_flags = mnt_flags; + } + br_write_unlock(&vfsmount_lock); + up_read(&sb->s_umount); + } else if (!capable(CAP_SYS_ADMIN)) err = -EPERM; - else + else { + down_write(&sb->s_umount); err = do_remount_sb(sb, flags, data, 0); - if (!err) { - br_write_lock(&vfsmount_lock); - mnt_flags |= mnt->mnt.mnt_flags & MNT_PROPAGATION_MASK; - mnt->mnt.mnt_flags = mnt_flags; - br_write_unlock(&vfsmount_lock); + if (!err) { + br_write_lock(&vfsmount_lock); + mnt_flags |= mnt->mnt.mnt_flags & MNT_PROPAGATION_MASK; + mnt->mnt.mnt_flags = mnt_flags; + br_write_unlock(&vfsmount_lock); + } + up_write(&sb->s_umount); } - up_write(&sb->s_umount); if (!err) { br_write_lock(&vfsmount_lock); touch_mnt_namespace(mnt->mnt_ns); -- 1.8.4.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
