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

[Aditya Kali]

Hi Al and other fs-developers,

Please let me know what you think about this patch.

Thanks,
-- 
Aditya

On Thu, Sep 19, 2013 at 1:13 PM, Aditya Kali <adityakali@xxxxxxxxxx> wrote:
>
>
> 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
>
>
>
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