Re: [tip:locking/core] locking/pvqspinlock: Replace xchg() by the more descriptive set_mb()

Waiman Long <waiman.long@xxxxxx> · Mon, 11 May 2015 12:50:29 -0400

On 05/11/2015 10:54 AM, Peter Zijlstra wrote:

On Mon, May 11, 2015 at 05:43:55AM -0700, tip-bot for Waiman Long wrote:

-		(void)xchg(&pn->state, vcpu_halted);
+		set_mb(pn->state, vcpu_halted);

Hmm, so I looked at the set_mb() definitions and I figure we want to do
something like the below, right?



Yes, I think we should do that just to be safe from unexpected compiler 

optimization.



---
  arch/arm/include/asm/barrier.h      | 2 +-
  arch/arm64/include/asm/barrier.h    | 2 +-
  arch/ia64/include/asm/barrier.h     | 2 +-
  arch/metag/include/asm/barrier.h    | 2 +-
  arch/mips/include/asm/barrier.h     | 2 +-
  arch/powerpc/include/asm/barrier.h  | 2 +-
  arch/s390/include/asm/barrier.h     | 2 +-
  arch/sparc/include/asm/barrier_64.h | 2 +-
  arch/x86/include/asm/barrier.h      | 2 +-
  include/asm-generic/barrier.h       | 2 +-
  10 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/arch/arm/include/asm/barrier.h b/arch/arm/include/asm/barrier.h
index d2f81e6b8c1c..993150aea681 100644
--- a/arch/arm/include/asm/barrier.h
+++ b/arch/arm/include/asm/barrier.h
@@ -81,7 +81,7 @@ do {									\
  #define read_barrier_depends()		do { } while(0)
  #define smp_read_barrier_depends()	do { } while(0)

-#define set_mb(var, value)	do { var = value; smp_mb(); } while (0)
+#define set_mb(var, value)	do { WRITE_ONCE(var, value); smp_mb(); } while (0)

  #define smp_mb__before_atomic()	smp_mb()
  #define smp_mb__after_atomic()	smp_mb()
diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index 71f19c4dc0de..ff7de78d01b8 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -114,7 +114,7 @@ do {									\
  #define read_barrier_depends()		do { } while(0)
  #define smp_read_barrier_depends()	do { } while(0)

-#define set_mb(var, value)	do { var = value; smp_mb(); } while (0)
+#define set_mb(var, value)	do { WRITE_ONCE(var, value); smp_mb(); } while (0)
  #define nop()		asm volatile("nop");

  #define smp_mb__before_atomic()	smp_mb()
diff --git a/arch/ia64/include/asm/barrier.h b/arch/ia64/include/asm/barrier.h
index f6769eb2bbf9..03117e7b2ab8 100644
--- a/arch/ia64/include/asm/barrier.h
+++ b/arch/ia64/include/asm/barrier.h
@@ -82,7 +82,7 @@ do {									\
   * acquire vs release semantics but we can't discuss this stuff with
   * Linus just yet.  Grrr...
   */
-#define set_mb(var, value)	do { (var) = (value); mb(); } while (0)
+#define set_mb(var, value)	do { WRITE_ONCE(var, value); mb(); } while (0)

  /*
   * The group barrier in front of the rsm&  ssm are necessary to ensure
diff --git a/arch/metag/include/asm/barrier.h b/arch/metag/include/asm/barrier.h
index d703d8e26a65..97eb018a2933 100644
--- a/arch/metag/include/asm/barrier.h
+++ b/arch/metag/include/asm/barrier.h
@@ -84,7 +84,7 @@ static inline void fence(void)
  #define read_barrier_depends()		do { } while (0)
  #define smp_read_barrier_depends()	do { } while (0)

-#define set_mb(var, value) do { var = value; smp_mb(); } while (0)
+#define set_mb(var, value) do { WRITE_ONCE(var, value); smp_mb(); } while (0)

  #define smp_store_release(p, v)						\
  do {									\
diff --git a/arch/mips/include/asm/barrier.h b/arch/mips/include/asm/barrier.h
index 2b8bbbcb9be0..cff1bbdaa74a 100644
--- a/arch/mips/include/asm/barrier.h
+++ b/arch/mips/include/asm/barrier.h
@@ -113,7 +113,7 @@
  #endif

  #define set_mb(var, value) \
-	do { var = value; smp_mb(); } while (0)
+	do { WRITE_ONCE(var, value); smp_mb(); } while (0)

  #define smp_llsc_mb()	__asm__ __volatile__(__WEAK_LLSC_MB : : :"memory")

diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
index a3bf5be111ff..2a072e48780d 100644
--- a/arch/powerpc/include/asm/barrier.h
+++ b/arch/powerpc/include/asm/barrier.h
@@ -34,7 +34,7 @@
  #define rmb()  __asm__ __volatile__ ("sync" : : : "memory")
  #define wmb()  __asm__ __volatile__ ("sync" : : : "memory")

-#define set_mb(var, value)	do { var = value; mb(); } while (0)
+#define set_mb(var, value)	do { WRITE_ONCE(var, value); mb(); } while (0)

  #ifdef __SUBARCH_HAS_LWSYNC
  #    define SMPWMB      LWSYNC
diff --git a/arch/s390/include/asm/barrier.h b/arch/s390/include/asm/barrier.h
index 8d724718ec21..b66cd53d35fc 100644
--- a/arch/s390/include/asm/barrier.h
+++ b/arch/s390/include/asm/barrier.h
@@ -36,7 +36,7 @@
  #define smp_mb__before_atomic()		smp_mb()
  #define smp_mb__after_atomic()		smp_mb()

-#define set_mb(var, value)		do { var = value; mb(); } while (0)
+#define set_mb(var, value)		do { WRITE_ONCE(var, value); mb(); } while (0)

  #define smp_store_release(p, v)						\
  do {									\
diff --git a/arch/sparc/include/asm/barrier_64.h b/arch/sparc/include/asm/barrier_64.h
index 76648941fea7..125fec7512f4 100644
--- a/arch/sparc/include/asm/barrier_64.h
+++ b/arch/sparc/include/asm/barrier_64.h
@@ -41,7 +41,7 @@ do {	__asm__ __volatile__("ba,pt	%%xcc, 1f\n\t" \
  #define dma_wmb()	wmb()

  #define set_mb(__var, __value) \
-	do { __var = __value; membar_safe("#StoreLoad"); } while(0)
+	do { WRITE_ONCE(__var, __value); membar_safe("#StoreLoad"); } while(0)

  #ifdef CONFIG_SMP
  #define smp_mb()	mb()
diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index 959e45b81fe2..9de5cde133a1 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -40,7 +40,7 @@
  #define smp_mb()	barrier()
  #define smp_rmb()	barrier()
  #define smp_wmb()	barrier()
-#define set_mb(var, value) do { var = value; barrier(); } while (0)
+#define set_mb(var, value) do { WRITE_ONCE(var, value); barrier(); } while (0)


That part is in the !CONFIG_SMP portion. I don't think we need to change it.

Cheers,
Longman
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