linux-kernel - [PATCH] arch/powerpc/include/asm/barrier.h: redefine rmb and wmb to lwsync

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Date:   Wed, 22 Feb 2023 11:31:07 +0530
From:   Kautuk Consul <kconsul@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Kautuk Consul <kconsul@...ux.vnet.ibm.com>
Subject: [PATCH] arch/powerpc/include/asm/barrier.h: redefine rmb and wmb to lwsync

A link from ibm.com states:
"Ensures that all instructions preceding the call to __lwsync
 complete before any subsequent store instructions can be executed
 on the processor that executed the function. Also, it ensures that
 all load instructions preceding the call to __lwsync complete before
 any subsequent load instructions can be executed on the processor
 that executed the function. This allows you to synchronize between
 multiple processors with minimal performance impact, as __lwsync
 does not wait for confirmation from each processor."

Thats why smp_rmb() and smp_wmb() are defined to lwsync.
But this same understanding applies to parallel pipeline
execution on each PowerPC processor.
So, use the lwsync instruction for rmb() and wmb() on the PPC
architectures that support it.

Also removed some useless spaces.

Signed-off-by: Kautuk Consul <kconsul@...ux.vnet.ibm.com>
---
 arch/powerpc/include/asm/barrier.h | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
index e80b2c0e9315..553f5a5d20bd 100644
--- a/arch/powerpc/include/asm/barrier.h
+++ b/arch/powerpc/include/asm/barrier.h
@@ -41,11 +41,17 @@

 /* The sub-arch has lwsync */
 #if defined(CONFIG_PPC64) || defined(CONFIG_PPC_E500MC)
-#    define SMPWMB      LWSYNC
+#undef rmb
+#undef wmb
+/* Redefine rmb() to lwsync. */
+#define rmb()	({__asm__ __volatile__ ("lwsync" : : : "memory"); })
+/* Redefine wmb() to lwsync. */
+#define wmb()	({__asm__ __volatile__ ("lwsync" : : : "memory"); })
+#define SMPWMB      LWSYNC
 #elif defined(CONFIG_BOOKE)
-#    define SMPWMB      mbar
+#define SMPWMB      mbar
 #else
-#    define SMPWMB      eieio
+#define SMPWMB      eieio
 #endif

 /* clang defines this macro for a builtin, which will not work with runtime patching */
-- 
2.31.1