Re: [PATCH v4 00/11] Rework READ_ONCE() to improve codegen

[Will Deacon]

On Fri, Apr 24, 2020 at 05:54:10PM +0200, Marco Elver wrote:
> On Fri, 24 Apr 2020 at 15:42, Will Deacon <will@xxxxxxxxxx> wrote:
> > On Wed, Apr 22, 2020 at 01:26:27PM +0100, Will Deacon wrote:
> > > On Wed, Apr 22, 2020 at 01:37:21PM +0200, Peter Zijlstra wrote:
> > > > So I'm obviously all for these patches; do note however that it collides
> > > > most mighty with the KCSAN stuff, which I believe is still pending.
> > >
> > > That stuff has been pending for the last two releases afaict :/
> > >
> > > Anyway, I'm happy to either provide a branch with this series on, or do
> > > the merge myself, or send this again based on something else. What works
> > > best for you? The only thing I'd obviously like to avoid is tightly
> > > coupling this to KCSAN if there's a chance of it missing the merge window
> > > again.
> >
> > FWIW, I had a go at rebasing onto linux-next, just to get an idea for how
> > bad it is. It's fairly bad, and I don't think it's fair to inflict it on
> > sfr. I've included the interesting part of the resulting compiler.h below
> > for you and the KCSAN crowd to take a look at (yes, there's room for
> > subsequent cleanup, but I was focussing on the conflict resolution for now).
> 
> Thanks for the heads up. From what I can tell, your proposed change
> may work fine for KCSAN. However, I've had trouble compiling this:
> 
> 1. kcsan_disable_current() / kcsan_enable_current() do not work as-is,
> because READ_ONCE/WRITE_ONCE seems to be used from compilation units
> where the KCSAN runtime is not available (e.g.
> arch/x86/entry/vdso/Makefile which had to set KCSAN_SANITIZE := n for
> that reason).
> 2. Some new uaccess whitelist entries were needed.
> 
> I think this is what's needed:
> https://lkml.kernel.org/r/20200424154730.190041-1-elver@xxxxxxxxxx
> 
> With that you can change the calls to __kcsan_disable_current() /
> __kcsan_enable_current() for READ_ONCE() and WRITE_ONCE(). After that,
> I was able to compile, and my test suite passed.

Brill, thanks Marco! I'll take a look at your other patches, but I'm pretty
stuck until we've figured out the merging plan for all of this.

Will