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Message-Id: <20190613134932.954074997@infradead.org>
Date: Thu, 13 Jun 2019 15:43:19 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: stern@...land.harvard.edu, akiyks@...il.com,
andrea.parri@...rulasolutions.com, boqun.feng@...il.com,
dlustig@...dia.com, dhowells@...hat.com, j.alglave@....ac.uk,
luc.maranget@...ia.fr, npiggin@...il.com, paulmck@...ux.ibm.com,
peterz@...radead.org, will.deacon@....com, paul.burton@...s.com
Cc: linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
Huacai Chen <chenhc@...ote.com>,
Huang Pei <huangpei@...ngson.cn>
Subject: [PATCH v2 2/4] mips/atomic: Fix loongson_llsc_mb() wreckage
The comment describing the loongson_llsc_mb() reorder case doesn't
make any sense what so ever. Instruction re-ordering is not an SMP
artifact, but rather a CPU local phenomenon. Clarify the comment by
explaining that these issue cause a coherence fail.
For the branch speculation case; if futex_atomic_cmpxchg_inatomic()
needs one at the bne branch target, then surely the normal
__cmpxch_asm() implementation does too. We cannot rely on the
barriers from cmpxchg() because cmpxchg_local() is implemented with
the same macro, and branch prediction and speculation are, too, CPU
local.
Fixes: e02e07e3127d ("MIPS: Loongson: Introduce and use loongson_llsc_mb()")
Cc: Huacai Chen <chenhc@...ote.com>
Cc: Huang Pei <huangpei@...ngson.cn>
Cc: Paul Burton <paul.burton@...s.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
arch/mips/include/asm/atomic.h | 5 +++--
arch/mips/include/asm/barrier.h | 32 ++++++++++++++++++--------------
arch/mips/include/asm/bitops.h | 5 +++++
arch/mips/include/asm/cmpxchg.h | 5 +++++
arch/mips/kernel/syscall.c | 1 +
5 files changed, 32 insertions(+), 16 deletions(-)
--- a/arch/mips/include/asm/atomic.h
+++ b/arch/mips/include/asm/atomic.h
@@ -193,6 +193,7 @@ static __inline__ int atomic_sub_if_posi
if (kernel_uses_llsc) {
int temp;
+ loongson_llsc_mb();
__asm__ __volatile__(
" .set push \n"
" .set "MIPS_ISA_LEVEL" \n"
@@ -200,12 +201,12 @@ static __inline__ int atomic_sub_if_posi
" .set pop \n"
" subu %0, %1, %3 \n"
" move %1, %0 \n"
- " bltz %0, 1f \n"
+ " bltz %0, 2f \n"
" .set push \n"
" .set "MIPS_ISA_LEVEL" \n"
" sc %1, %2 \n"
"\t" __scbeqz " %1, 1b \n"
- "1: \n"
+ "2: \n"
" .set pop \n"
: "=&r" (result), "=&r" (temp),
"+" GCC_OFF_SMALL_ASM() (v->counter)
--- a/arch/mips/include/asm/barrier.h
+++ b/arch/mips/include/asm/barrier.h
@@ -238,36 +238,40 @@
/*
* Some Loongson 3 CPUs have a bug wherein execution of a memory access (load,
- * store or pref) in between an ll & sc can cause the sc instruction to
+ * 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.
+ * 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 & any earlier
- * memory access instructions. Many of these cases are already covered by
- * smp_mb__before_llsc() but for the remaining cases, typically ones in
- * which multiple CPUs may operate on a memory location but ordering is not
- * usually guaranteed, we use loongson_llsc_mb() below.
+ * 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()
+ * 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.
+ * 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)
+ * 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__(__WEAK_LLSC_MB : : :"memory")
+#define loongson_llsc_mb() __asm__ __volatile__("sync" : : :"memory")
#else
#define loongson_llsc_mb() do { } while (0)
#endif
--- a/arch/mips/include/asm/bitops.h
+++ b/arch/mips/include/asm/bitops.h
@@ -249,6 +249,7 @@ static inline int test_and_set_bit(unsig
unsigned long *m = ((unsigned long *) addr) + (nr >> SZLONG_LOG);
unsigned long temp;
+ loongson_llsc_mb();
do {
__asm__ __volatile__(
" .set push \n"
@@ -305,6 +306,7 @@ static inline int test_and_set_bit_lock(
unsigned long *m = ((unsigned long *) addr) + (nr >> SZLONG_LOG);
unsigned long temp;
+ loongson_llsc_mb();
do {
__asm__ __volatile__(
" .set push \n"
@@ -364,6 +366,7 @@ static inline int test_and_clear_bit(uns
unsigned long *m = ((unsigned long *) addr) + (nr >> SZLONG_LOG);
unsigned long temp;
+ loongson_llsc_mb();
do {
__asm__ __volatile__(
" " __LL "%0, %1 # test_and_clear_bit \n"
@@ -379,6 +382,7 @@ static inline int test_and_clear_bit(uns
unsigned long *m = ((unsigned long *) addr) + (nr >> SZLONG_LOG);
unsigned long temp;
+ loongson_llsc_mb();
do {
__asm__ __volatile__(
" .set push \n"
@@ -438,6 +442,7 @@ static inline int test_and_change_bit(un
unsigned long *m = ((unsigned long *) addr) + (nr >> SZLONG_LOG);
unsigned long temp;
+ loongson_llsc_mb();
do {
__asm__ __volatile__(
" .set push \n"
--- a/arch/mips/include/asm/cmpxchg.h
+++ b/arch/mips/include/asm/cmpxchg.h
@@ -46,6 +46,7 @@ extern unsigned long __xchg_called_with_
__typeof(*(m)) __ret; \
\
if (kernel_uses_llsc) { \
+ loongson_llsc_mb(); \
__asm__ __volatile__( \
" .set push \n" \
" .set noat \n" \
@@ -117,6 +118,7 @@ static inline unsigned long __xchg(volat
__typeof(*(m)) __ret; \
\
if (kernel_uses_llsc) { \
+ loongson_llsc_mb(); \
__asm__ __volatile__( \
" .set push \n" \
" .set noat \n" \
@@ -134,6 +136,7 @@ static inline unsigned long __xchg(volat
: "=&r" (__ret), "=" GCC_OFF_SMALL_ASM() (*m) \
: GCC_OFF_SMALL_ASM() (*m), "Jr" (old), "Jr" (new) \
: "memory"); \
+ loongson_llsc_mb(); \
} else { \
unsigned long __flags; \
\
@@ -229,6 +232,7 @@ static inline unsigned long __cmpxchg64(
*/
local_irq_save(flags);
+ loongson_llsc_mb();
asm volatile(
" .set push \n"
" .set " MIPS_ISA_ARCH_LEVEL " \n"
@@ -274,6 +278,7 @@ static inline unsigned long __cmpxchg64(
"r" (old),
"r" (new)
: "memory");
+ loongson_llsc_mb();
local_irq_restore(flags);
return ret;
--- a/arch/mips/kernel/syscall.c
+++ b/arch/mips/kernel/syscall.c
@@ -132,6 +132,7 @@ static inline int mips_atomic_set(unsign
[efault] "i" (-EFAULT)
: "memory");
} else if (cpu_has_llsc) {
+ loongson_llsc_mb();
__asm__ __volatile__ (
" .set push \n"
" .set "MIPS_ISA_ARCH_LEVEL" \n"
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