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SubjectRe: [PATCH -v3 7/8] locking: Move smp_cond_load_acquire() and friends into asm-generic/barrier.h
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@infradead.org>
>
> --- 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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