Messages in this thread | | | From | Aditya Kali <> | Date | Mon, 30 Sep 2013 14:44:33 -0700 | Subject | Re: [RFC] vfs: avoid sb->s_umount lock while changing bind-mount flags |
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On Mon, Sep 30, 2013 at 1:03 PM, Al Viro <viro@zeniv.linux.org.uk> wrote: > On Mon, Sep 30, 2013 at 11:13:23AM -0700, Aditya Kali wrote: >> +Ted Ts'o, Tejun Heo, Jens Axboe >> >> >> On 09/30/2013 10:54 AM, Aditya Kali wrote: >> >Hi Al and other fs-developers, >> > >> >Please let me know what you think about this patch. > > Don't top-post, please... What prevents a race between MS_BIND remounts > and plain ones? Used to be serialized on ->s_umount, but...
The rational here is that MS_BIND remounts need to act on the bind mount-point only (struct vfsmount *) and not on the underlying superblock (struct super_block *). So, intuitively, it should not be necessary to take a superblock level lock for the MS_BIND case. As for the modification of mnt_flags on vfsmount, there is already vfsmount_lock taken in both MS_BIND and non-MS_BIND case to prevent the race.
Following is the example that demonstrates the problem that I am trying to address with this patch:
(1) /dev/sda is mounted at /mnt/sda (2) /mnt/sda/users/user1 is bind mounted at /home/user1/ ; /mnt/sda/users/user2 is bind mounted at /home/user2/ (3) user1 is doing buffered writes in /home/user1/logs/ (4) admin tries to make /home/user2/bin/ read-only using a bind-mounts: $ mount --bind /home/user2/bin/ /home/user2/bin/ # this is fast $ mount --bind -o remount,ro /home/user2/bin/ # this blocks behind any writeback happening on sda (because of sb->s_umount write lock)
Please advise.
Thanks, -- Aditya
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