On Thu, Jul 2, 2020 at 11:48 AM Will Deacon [off-list ref] wrote:
On Thu, Jul 02, 2020 at 10:32:39AM +0100, Mark Rutland wrote:
quoted
On Tue, Jun 30, 2020 at 06:37:20PM +0100, Will Deacon wrote:
quoted
-#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
I also have a question that I didn't dare ask when the same
code came up before (I guess it's also what's in the kernel today):
With the cast to 'typeof(*p)' at the end, doesn't that mean we
lose the effect of __unqual_scalar_typeof() again, so any "volatile"
pointer passed into __READ_ONCE_SCALAR() or
__smp_load_acquire() still leads to a volatile load of the original
variable, plus another volatile access to ___p1 after
spilling it to the stack as a non-volatile variable?
I hope I'm missing something obvious here.
Arnd