Re: [PATCH 0/5] USB: fix tty unthrottle races

[Johan Hovold <johan@kernel.org>]

On Thu, Apr 25, 2019 at 04:58:20PM -0400, Alan Stern wrote:
> On Thu, 25 Apr 2019, Johan Hovold wrote:
> 
> > This series fixes a couple of long-standing issues in USB serial and
> > cdc-acm which essentially share the same implementation.
> > 
> > As noted by Oliver a few years back, read-urb completion can race with
> > unthrottle() running on another CPU and this can potentially lead to
> > memory corruption. This particular bug in cdc-acm was unfortunately
> > reintroduced a year later.
> > 
> > There's also a second race due to missing memory barriers which could
> > theoretically lead to the port staying throttled until reopened on
> > weakly ordered systems. A second set of memory barriers should address
> > that.
> > 
> > I would appreciate your keen eyes on this one to make sure I got the
> > barriers right.
> > 
> > I noticed there's some on-going discussion about the atomic memory
> > barriers that Alan's involved in, and I'll try to catch up on his
> > data-race work as well. I'm still a little concerned about whether the
> > smp_mb__before_atomic() is sufficient to prevent the compiler from
> > messing things up without adding READ_ONCE().
> 
> I think your changes in patches 1 and 4 are correct.  Regardless of the
> issues still undergoing discussion elsewhere, smp_mb__before_atomic()
> should cause both the compiler and the CPU to order every preceding
> operation (READ_ONCE or not) before the atomic op.

Ok, thanks for taking a look!

Johan