linux-kernel - Re: Does Itanium permit speculative stores?

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Date:	Tue, 12 Nov 2013 19:34:04 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Does Itanium permit speculative stores?

On Tue, Nov 12, 2013 at 06:00:26PM +0000, Luck, Tony wrote:
> The "ACCESS_ONCE" macro casts to volatile - which will make gcc generate
> ordered "ld.acq" and "st.rel" instructions for your code snippets. So I think
> you should be fine.

Hurm.. so:

+#define smp_store_release(p, v)						\
+do {									\
+	compiletime_assert_atomic_type(*p);				\
+	switch (sizeof(*p)) {						\
+	case 4:								\
+		asm volatile ("st4.rel [%0]=%1"				\
+				: "=r" (p) : "r" (v) : "memory");	\
+		break;							\
+	case 8:								\
+		asm volatile ("st8.rel [%0]=%1"				\
+				: "=r" (p) : "r" (v) : "memory");	\
+		break;							\
+	}								\
+} while (0)
+
+#define smp_load_acquire(p)						\
+({									\
+	typeof(*p) ___p1;						\
+	compiletime_assert_atomic_type(*p);				\
+	switch (sizeof(*p)) {						\
+	case 4:								\
+		asm volatile ("ld4.acq %0=[%1]"				\
+				: "=r" (___p1) : "r" (p) : "memory");	\
+		break;							\
+	case 8:								\
+		asm volatile ("ld8.acq %0=[%1]"				\
+				: "=r" (___p1) : "r" (p) : "memory");	\
+		break;							\
+	}								\
+	___p1;								\
+})

That all can be written as:

+#define smp_store_release(p, v)						\
+do {									\
+	compiletime_assert_atomic_type(*p);				\
+	ACCESS_ONCE(*p) = (v);						\
+} while (0)
+
+#define smp_load_acquire(p)						\
+({									\
+	typeof(*p) ___p1 = ACCESS_ONCE(*p);				\
+	compiletime_assert_atomic_type(*p);				\
+	___p1;								\
+})

On ia64? Totally much simpler!
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