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SubjectRe: [powerpc/next, 1/2] powerpc: Make value-returning atomics fully ordered
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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: <stable@vger.kernel.org> # 3.4+
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Applied to powerpc next, thanks.

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

cheers


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