Re: [PATCH 17/17] RCU'd vfsmounts

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Oct 03, 2013 at 01:52:45PM -0700, Linus Torvalds wrote:

> Yeah, I think we should be guaranteed that, because the
> synchronize_rcu() will guarantee that all other CPU's go through an
> idle period. So the "read A" on CPU2 cannot possibly see a 1 _unless_
> it happens so early that synchronize_rcu() definitely sees it (ie it's
> a "preexisting reader" by definition), in which case synchronize_rcu()
> will be waiting for a subsequent idle period, in which case the B=0 on
> CPU2 is not only guaranteed to happen but also be visible out, so the
> "read B" on CPU1 will see 0. And that's true even if CPU2 doesn't have
> an explicit memory barrier, because the "RCU idle" state implies that
> it has gone through a barrier.
> 
> So I don't see how they could possibly see ones. Modulo terminal bugs
> in synchronize_barrier() (which can be very slow, but for umount I
> wouldn't worry). Or modulo my brain being fried.

There's one more place similar to that - kern_unmount().  There we also
go from "longterm vfsmount, mntput() doesn't need to bother checking"
to NULL ->mnt_ns.  We can, of course, slap synchronize_rcu() there as
well, but that might make pid_ns and ipc_ns destruction slow...