Subject: Re: [powerpc/next, 1/2] powerpc: Make value-returning atomics fully ordered

From: Michael Ellerman <[email-masked]>
Date: Tue, 15 Dec 2015 22:27:12 +1100 (AEDT)

On Mon, 2015-02-11 at 01:30:31 UTC, Boqun Feng wrote:
> According to memory-barriers.txt:
> 
> > Any atomic operation that modifies some state in memory and returns
> > information about the state (old or new) implies an SMP-conditional
> > general memory barrier (smp_mb()) on each side of the actual
> > operation ...
> 
> Which mean these operations should be fully ordered. However on PPC,
> PPC_ATOMIC_ENTRY_BARRIER is the barrier before the actual operation,
> which is currently "lwsync" if SMP=y. The leading "lwsync" can not
> guarantee fully ordered atomics, according to Paul Mckenney:
> 
> https://lkml.org/lkml/2015/10/14/970
> 
> To fix this, we define PPC_ATOMIC_ENTRY_BARRIER as "sync" to guarantee
> the fully-ordered semantics.
> 
> This also makes futex atomics fully ordered, which can avoid possible
> memory ordering problems if userspace code relies on futex system call
> for fully ordered semantics.
> 
> Cc: <[email-masked]> # 3.4+
> Signed-off-by: Boqun Feng <[email-masked]>
> Reviewed-by: Paul E. McKenney <[email-masked]>
> Acked-by: Peter Zijlstra (Intel) <[email-masked]>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/49e9cf3f0c04bf76ffa59242

cheers