On 05/31/2016 05:41 AM, Peter Zijlstra wrote:
Since all asm/barrier.h should/must include asm-generic/barrier.h the
latter is a good place for generic infrastructure like this.
This also allows archs to override the new
smp_acquire__after_ctrl_dep().
Signed-off-by: Peter Zijlstra (Intel)<peterz@xxxxxxxxxxxxx>
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -194,7 +194,7 @@ do { \
})
#endif
-#endif
+#endif /* CONFIG_SMP */
/* Barriers for virtual machine guests when talking to an SMP host */
#define virt_mb() __smp_mb()
@@ -207,5 +207,61 @@ do { \
#define virt_store_release(p, v) __smp_store_release(p, v)
#define virt_load_acquire(p) __smp_load_acquire(p)
+/**
+ * smp_acquire__after_ctrl_dep() - Provide ACQUIRE ordering after a control dependency
+ *
+ * A control dependency provides a LOAD->STORE order, the additional RMB
+ * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
+ * aka. (load)-ACQUIRE.
+ *
+ * Architectures that do not do load speculation can have this be barrier().
+ */
+#ifndef smp_acquire__after_ctrl_dep
+#define smp_acquire__after_ctrl_dep() smp_rmb()
+#endif
+
+/**
+ * cmpwait - compare and wait for a variable to change
+ * @ptr: pointer to the variable to wait on
+ * @val: the value it should change from
+ *
+ * A simple constuct that waits for a variable to change from a known
+ * value; some architectures can do this in hardware.
+ */
+#ifndef cmpwait
+#define cmpwait(ptr, val) do { \
+ typeof (ptr) __ptr = (ptr); \
+ typeof (val) __val = (val); \
+ while (READ_ONCE(*__ptr) == __val) \
+ cpu_relax(); \
+} while (0)
+#endif
+
+/**
+ * smp_cond_load_acquire() - (Spin) wait for cond with ACQUIRE ordering
+ * @ptr: pointer to the variable to wait on
+ * @cond: boolean expression to wait for
+ *
+ * Equivalent to using smp_load_acquire() on the condition variable but employs
+ * the control dependency of the wait to reduce the barrier on many platforms.
+ *
+ * Due to C lacking lambda expressions we load the value of *ptr into a
+ * pre-named variable @VAL to be used in @cond.
+ */
+#ifndef smp_cond_load_acquire
+#define smp_cond_load_acquire(ptr, cond_expr) ({ \
+ typeof(ptr) __PTR = (ptr); \
+ typeof(*ptr) VAL; \
+ for (;;) { \
+ VAL = READ_ONCE(*__PTR); \
+ if (cond_expr) \
+ break; \
+ cmpwait(__PTR, VAL); \
+ } \
+ smp_acquire__after_ctrl_dep(); \
+ VAL; \
+})
+#endif
You are doing two READ_ONCE's in the smp_cond_load_acquire loop. Can we
change it to do just one READ_ONCE, like
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -229,12 +229,18 @@ do {
* value; some architectures can do this in hardware.
*/
#ifndef cmpwait
-#define cmpwait(ptr, val) do { \
+#define cmpwait(ptr, val) ({ \
typeof (ptr) __ptr = (ptr); \
- typeof (val) __val = (val); \
- while (READ_ONCE(*__ptr) == __val) \
+ typeof (val) __old = (val); \
+ typeof (val) __new; \
+ for (;;) { \
+ __new = READ_ONCE(*__ptr); \
+ if (__new != __old) \
+ break; \
cpu_relax(); \
-} while (0)
+ } \
+ __new; \
+})
#endif
/**
@@ -251,12 +257,11 @@ do {
#ifndef smp_cond_load_acquire
#define smp_cond_load_acquire(ptr, cond_expr) ({ \
typeof(ptr) __PTR = (ptr); \
- typeof(*ptr) VAL; \
+ typeof(*ptr) VAL = READ_ONCE(*__PTR); \
for (;;) { \
- VAL = READ_ONCE(*__PTR); \
if (cond_expr) \
break; \
- cmpwait(__PTR, VAL); \
+ VAL = cmpwait(__PTR, VAL); \
} \
smp_acquire__after_ctrl_dep(); \
VAL; \
Cheers,
Longman
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