linux-kernel - [PATCH v2 32/36] MIPS: barrier: Remove loongson_llsc

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Message-ID: <20191001215249.4157062-33-paul.burton@mips.com>
Date:   Tue, 1 Oct 2019 21:53:40 +0000
From:   Paul Burton <paul.burton@...s.com>
To:     "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>
CC:     Huacai Chen <chenhc@...ote.com>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Paul Burton <pburton@...ecomp.com>
Subject: [PATCH v2 32/36] MIPS: barrier: Remove loongson_llsc_mb()

The loongson_llsc_mb() macro is no longer used - instead barriers are
emitted as part of inline asm using the __SYNC() macro. Remove the
now-defunct loongson_llsc_mb() macro.

Signed-off-by: Paul Burton <paul.burton@...s.com>
---

Changes in v2: None

 arch/mips/include/asm/barrier.h | 40 ---------------------------------
 arch/mips/loongson64/Platform   |  2 +-
 2 files changed, 1 insertion(+), 41 deletions(-)

diff --git a/arch/mips/include/asm/barrier.h b/arch/mips/include/asm/barrier.h
index 133afd565067..6d92d5ccdafa 100644
--- a/arch/mips/include/asm/barrier.h
+++ b/arch/mips/include/asm/barrier.h
@@ -122,46 +122,6 @@ static inline void wmb(void)
 #define __smp_mb__before_atomic()	__smp_mb__before_llsc()
 #define __smp_mb__after_atomic()	smp_llsc_mb()
 
-/*
- * Some Loongson 3 CPUs have a bug wherein execution of a memory access (load,
- * store or prefetch) in between an LL & SC can cause the SC instruction to
- * erroneously succeed, breaking atomicity. Whilst it's unusual to write code
- * containing such sequences, this bug bites harder than we might otherwise
- * expect due to reordering & speculation:
- *
- * 1) A memory access appearing prior to the LL in program order may actually
- *    be executed after the LL - this is the reordering case.
- *
- *    In order to avoid this we need to place a memory barrier (ie. a SYNC
- *    instruction) prior to every LL instruction, in between it and any earlier
- *    memory access instructions.
- *
- *    This reordering case is fixed by 3A R2 CPUs, ie. 3A2000 models and later.
- *
- * 2) If a conditional branch exists between an LL & SC with a target outside
- *    of the LL-SC loop, for example an exit upon value mismatch in cmpxchg()
- *    or similar, then misprediction of the branch may allow speculative
- *    execution of memory accesses from outside of the LL-SC loop.
- *
- *    In order to avoid this we need a memory barrier (ie. a SYNC instruction)
- *    at each affected branch target, for which we also use loongson_llsc_mb()
- *    defined below.
- *
- *    This case affects all current Loongson 3 CPUs.
- *
- * The above described cases cause an error in the cache coherence protocol;
- * such that the Invalidate of a competing LL-SC goes 'missing' and SC
- * erroneously observes its core still has Exclusive state and lets the SC
- * proceed.
- *
- * Therefore the error only occurs on SMP systems.
- */
-#ifdef CONFIG_CPU_LOONGSON3_WORKAROUNDS /* Loongson-3's LLSC workaround */
-#define loongson_llsc_mb()	__asm__ __volatile__("sync" : : :"memory")
-#else
-#define loongson_llsc_mb()	do { } while (0)
-#endif
-
 static inline void sync_ginv(void)
 {
 	asm volatile(__SYNC(ginv, always));
diff --git a/arch/mips/loongson64/Platform b/arch/mips/loongson64/Platform
index c1a4d4dc4665..28172500f95a 100644
--- a/arch/mips/loongson64/Platform
+++ b/arch/mips/loongson64/Platform
@@ -27,7 +27,7 @@ cflags-$(CONFIG_CPU_LOONGSON3)	+= -Wa,--trap
 #
 # Some versions of binutils, not currently mainline as of 2019/02/04, support
 # an -mfix-loongson3-llsc flag which emits a sync prior to each ll instruction
-# to work around a CPU bug (see loongson_llsc_mb() in asm/barrier.h for a
+# to work around a CPU bug (see __SYNC_loongson3_war in asm/sync.h for a
 # description).
 #
 # We disable this in order to prevent the assembler meddling with the
-- 
2.23.0