Re: [RFC] vfs: avoid sb->s_umount lock while changing bind-mount flags

[Aditya Kali <adityakali@google.com>]

On 09/16/2013 07:40 PM, Al Viro wrote:
> On Mon, Sep 16, 2013 at 10:42:30AM -0700, Aditya Kali wrote:
>> 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. Looking at the code it seems to me that
>> we need s_umount lock only around the do_remount_sb() call.
>> vfsmount_lock seems enough to protect the flag change on the mount.
>> So this patch fixes the locking so that changing of flags can happen
>> outside the down_write(&sb->s_umount).
>
> What's to prevent mount -o remount,ro /mnt and mount -o remount,rw,nodev /mnt
> racing and ending up with that sucker rw and without nodev?

Thanks for the reply! I see the problem in my patch. Please find the 
second attempt at this patch below. I have tried to keep the non-MS_BIND 
remount semantics same while moving the MS_BIND remount code outside of 
s_umount lock. Is it OK to not synchronize the non-MS_BIND 
do_remount_sb() call with change of mnt_flags in MS_BIND case?


---
  fs/namespace.c | 37 +++++++++++++++++++++++--------------
  1 file changed, 23 insertions(+), 14 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index da5c494..25c4faf 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)
@@ -1838,20 +1840,27 @@ 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) {
+		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);
+	} 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