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

[Al Viro]

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?  As for
lock_mount... nope - we carefully do *not* hold namespace_sem over any kind
of fs operations.  Anything getting stuck while holding it will have
really nasty consequences.

So ->s_umount here is inelegant, but alternatives sucks worse...
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