Re: [rfc][possible solution] RCU vfsmounts

[Linus Torvalds <torvalds@linux-foundation.org>]

On Sat, Sep 28, 2013 at 1:27 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
>         FWIW, I think I have a kinda-sorta solution for that and I'd like
> to hear your comments on that.  I want to replace vfsmount_lock with seqlock
> and store additional seq number in nameidata, set to vfsmount_seq in the
> beginning and rechecked in unlazy_walk/complete_walk.

Yes, that would be lovely.

>         The obvious variant would be to have unlazy_walk/complete_walk to
> grab refcount, check vfsmount_seq and mntput on mismatch.  The trouble
> with that is race with what would've been the final mntput() done by
> umount(2); complete_walk() would drop that temporary reference and
> fail, all right, but... we would get a umount(2) returning without having
> actually shut the filesystem down.  Said shutdown would happen in whoever
> had been doing pathname resolution that stepped into the race.

Sounds reasonable to to me.

Side note: I really wish there was some way to avoid having to
finalize the vfsmount entirely for some common things. For example,
"[l]stat[at]()" really doesn't need it for the common cases (network
filesystems may need to revalidate), and is a very critical operation,
and we *could* just look up the inode under RCU and never finalize the
dentry _or_ the vfsmount. However, very annoyingly, the security layer
wants the vfsmount, and we don't know if that is RCU-safe...

                Linus