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.h
diff --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")
quoted hunk ↗ jump to hunk
-#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())?
[...]
quoted hunk ↗ jump to hunk
+#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.