[PATCH v2 1/7] asm-generic: barrier: add smp_cond_load_relaxed_timewait()

[Ankur Arora]

Add smp_cond_load_relaxed_timewait(), which extends the non-timeout
variant for cases where we don't want to wait indefinitely.

The interface adds parameters to allow timeout checks and a policy
that decides how exactly to wait for the condition to change.

The waiting is done via the usual cpu_relax() spin-wait around the
conditional variable with periodic evaluation of the time-check
expression, and optionally by architectural primitives that allow
for cheaper mechanisms such as waiting on stores to a memory address
with an out-of-band timeout mechanism.

Cc: Arnd Bergmann <arnd at arndb.de>
Cc: Will Deacon <will at kernel.org>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Kumar Kartikeya Dwivedi <memxor at gmail.com>
Cc: Alexei Starovoitov <ast at kernel.org>
Cc: linux-arch at vger.kernel.org
Signed-off-by: Ankur Arora <ankur.a.arora at oracle.com>
---
 include/asm-generic/barrier.h | 58 +++++++++++++++++++++++++++++++++++
 1 file changed, 58 insertions(+)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index d4f581c1e21d..a7be98e906f4 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -273,6 +273,64 @@ do {									\
 })
 #endif
 
+/*
+ * Non-spin primitive that allows waiting for stores to an address,
+ * with support for a timeout. This works in conjunction with an
+ * architecturally defined wait_policy.
+ */
+#ifndef __smp_timewait_store
+#define __smp_timewait_store(ptr, val) do { } while (0)
+#endif
+
+#ifndef __smp_cond_load_relaxed_timewait
+#define __smp_cond_load_relaxed_timewait(ptr, cond_expr, wait_policy,	\
+					 time_expr, time_end) ({	\
+	typeof(ptr) __PTR = (ptr);					\
+	__unqual_scalar_typeof(*ptr) VAL;				\
+	u32 __n = 0, __spin = 0;					\
+	u64 __prev = 0, __end = (time_end);				\
+	bool __wait = false;						\
+									\
+	for (;;) {							\
+		VAL = READ_ONCE(*__PTR);				\
+		if (cond_expr)						\
+			break;						\
+		cpu_relax();						\
+		if (++__n < __spin)					\
+			continue;					\
+		if (!(__prev = wait_policy((time_expr), __prev, __end,	\
+					  &__spin, &__wait)))		\
+			break;						\
+		if (__wait)						\
+			__smp_timewait_store(__PTR, VAL);		\
+		__n = 0;						\
+	}								\
+	(typeof(*ptr))VAL;						\
+})
+#endif
+
+/**
+ * smp_cond_load_relaxed_timewait() - (Spin) wait for cond with no ordering
+ * guarantees until a timeout expires.
+ * @ptr: pointer to the variable to wait on
+ * @cond: boolean expression to wait for
+ * @wait_policy: policy handler that adjusts the number of times we spin or
+ *  wait for cacheline to change (depends on architecture, not supported in
+ *  generic code.) before evaluating the time-expr.
+ * @time_expr: monotonic expression that evaluates to the current time
+ * @time_end: compared against time_expr
+ *
+ * Equivalent to using READ_ONCE() on the condition variable.
+ */
+#define smp_cond_load_relaxed_timewait(ptr, cond_expr, wait_policy,	\
+					 time_expr, time_end) ({	\
+	__unqual_scalar_typeof(*ptr) _val;;				\
+	_val = __smp_cond_load_relaxed_timewait(ptr, cond_expr,		\
+					      wait_policy, time_expr,	\
+					      time_end);		\
+	(typeof(*ptr))_val;						\
+})
+
 /*
  * pmem_wmb() ensures that all stores for which the modification
  * are written to persistent storage by preceding instructions have
-- 
2.43.5