Re: [PATCH 04/18] alpha: Override READ_ONCE() with barriered implementation
From: Mark Rutland <mark.rutland@arm.com>
Date: 2020-07-02 09:32:54
Also in:
linux-alpha, lkml, virtualization
On Tue, Jun 30, 2020 at 06:37:20PM +0100, Will Deacon wrote:
quoted hunk ↗ jump to hunk
Rather then relying on the core code to use smp_read_barrier_depends() as part of the READ_ONCE() definition, instead override __READ_ONCE() in the Alpha code so that it is treated the same way as smp_load_acquire(). Acked-by: Paul E. McKenney <paulmck@kernel.org> Signed-off-by: Will Deacon <will@kernel.org> --- arch/alpha/include/asm/barrier.h | 61 ++++---------------------------- arch/alpha/include/asm/rwonce.h | 19 ++++++++++ 2 files changed, 26 insertions(+), 54 deletions(-) create mode 100644 arch/alpha/include/asm/rwonce.hdiff --git a/arch/alpha/include/asm/barrier.h b/arch/alpha/include/asm/barrier.h index 92ec486a4f9e..2ecd068d91d1 100644 --- a/arch/alpha/include/asm/barrier.h +++ b/arch/alpha/include/asm/barrier.h@@ -2,64 +2,17 @@ #ifndef __BARRIER_H #define __BARRIER_H -#include <asm/compiler.h> - #define mb() __asm__ __volatile__("mb": : :"memory") #define rmb() __asm__ __volatile__("mb": : :"memory") #define wmb() __asm__ __volatile__("wmb": : :"memory")
-#define read_barrier_depends() __asm__ __volatile__("mb": : :"memory")
+#define __smp_load_acquire(p) \
+({ \
+ __unqual_scalar_typeof(*p) ___p1 = \
+ (*(volatile typeof(___p1) *)(p)); \
+ compiletime_assert_atomic_type(*p); \
+ ___p1; \
+})
Sorry if I'm being thick, but doesn't this need a barrier after the
volatile access to provide the acquire semantic?
IIUC prior to this commit alpha would have used the asm-generic
__smp_load_acquire, i.e.
| #ifndef __smp_load_acquire
| #define __smp_load_acquire(p) \
| ({ \
| __unqual_scalar_typeof(*p) ___p1 = READ_ONCE(*p); \
| compiletime_assert_atomic_type(*p); \
| __smp_mb(); \
| (typeof(*p))___p1; \
| })
| #endif
... where the __smp_mb() would be alpha's mb() from earlier in the patch
context, i.e.
| #define mb() __asm__ __volatile__("mb": : :"memory")
... so don't we need similar before returning ___p1 above in
__smp_load_acquire() (and also matching the old read_barrier_depends())?
[...]
+#include <asm/barrier.h> + +/* + * Alpha is apparently daft enough to reorder address-dependent loads + * on some CPU implementations. Knock some common sense into it with + * a memory barrier in READ_ONCE(). + */ +#define __READ_ONCE(x) __smp_load_acquire(&(x))
As above, I don't see a memory barrier implied here, so this doesn't look quite right. Thanks, Mark. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel