On Wed, Apr 24, 2019 at 09:00:25PM +0000, Paul Burton wrote:
> Hi Peter,
>
> On Wed, Apr 24, 2019 at 02:36:57PM +0200, Peter Zijlstra wrote:
> > There were no memory barriers on cmpxchg64() _at_all_. Fix this.
>
> This does looks problematic, but it's worth noting that this code path
> is only applicable to 32b kernels running on 64b CPUs which is pretty
> rare. The commit message as-is suggests to me that all configurations
> are broken, which isn't the case (at least, not in this respect :) ).
OK, I hadn't gone through the ifdef selection process. I just
encountered this cmpxchg implementation and noted a significant lack of
barriers.
> >
> > Cc: Paul Burton <paul.bur...@mips.com>
> > Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
> > ---
> > arch/mips/include/asm/cmpxchg.h | 6 ++++--
> > 1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > --- a/arch/mips/include/asm/cmpxchg.h
> > +++ b/arch/mips/include/asm/cmpxchg.h
> > @@ -290,9 +290,11 @@ static inline unsigned long __cmpxchg64(
> > * will cause a build error unless cpu_has_64bits is a \
> > * compile-time constant 1. \
> > */ \
> > - if (cpu_has_64bits && kernel_uses_llsc) \
> > + if (cpu_has_64bits && kernel_uses_llsc) { \
> > + smp_mb__before_llsc(); \
> > __res = __cmpxchg64((ptr), __old, __new); \
> > - else \
> > + smp_llsc_mb(); \
> > + } else \
> > __res = __cmpxchg64_unsupported(); \
>
> It would be good to also add braces around the else block, and I believe
> checkpatch should be complaining about that ("braces {} should be used
> on all arms of this statement").
You're right, I'll fix up.
