[PATCH 1/4] asm-generic: barrier: Add smp_cond_load_relaxed_timewait()

[Ankur Arora]

Add smp_cond_load_relaxed_timewait(), a timed variant of
smp_cond_load_relaxed(). This is useful for cases where we want to
wait on a conditional variable but don't want to wait indefinitely.

The timeout is supported by extending the smp_cond_load_relaxed()
interface to specify a time-check expression and an associated
time-limit.

The generic version is implemented as the usual cpu_relax() spin-wait
loop with a periodic evaluation of the time-check expression. To
minimize the numbers of instructions executed in each iteration of
the loop, limit the frequency of the time-check to once every
smp_cond_time_check_count.
(The inner loop in poll_idle() has a substantially similar structure
and constraints as smp_cond_load_relaxed_timewait(), so we reuse the
same value for smp_cond_time_check_count.)

Architecture specific versions can implement this by waiting on a
cacheline to change and an out-of-band mechanism to allow for the
time check.

Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
Cc: linux-arch@xxxxxxxxxxxxxxx
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
 include/asm-generic/barrier.h | 48 +++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index d4f581c1e21d..31de8ed2a05e 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -273,6 +273,54 @@ do {									\
 })
 #endif
 
+#ifndef smp_cond_time_check_count
+/*
+ * Limit how often smp_cond_load_relaxed_timewait() evaluates time_expr_ns.
+ * This helps reduce the number of instructions executed while spin-waiting.
+ */
+#define smp_cond_time_check_count	200
+#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
+ * @time_expr_ns: evaluates to the current time
+ * @time_limit_ns: compared against time_expr_ns
+ *
+ * Equivalent to using READ_ONCE() on the condition variable.
+ *
+ * Note that the time check in time_expr_ns can be synchronous or
+ * asynchronous.
+ * In the generic version the check is synchronous but kept coarse
+ * to minimize instructions executed while spin-waiting.
+ */
+#ifndef __smp_cond_load_relaxed_spinwait
+#define __smp_cond_load_relaxed_spinwait(ptr, cond_expr, time_expr_ns,	\
+					 time_limit_ns) ({		\
+	typeof(ptr) __PTR = (ptr);					\
+	__unqual_scalar_typeof(*ptr) VAL;				\
+	unsigned int __count = 0;					\
+	for (;;) {							\
+		VAL = READ_ONCE(*__PTR);				\
+		if (cond_expr)						\
+			break;						\
+		cpu_relax();						\
+		if (__count++ < smp_cond_time_check_count)		\
+			continue;					\
+		if ((time_expr_ns) >= (time_limit_ns))			\
+			break;						\
+		__count = 0;						\
+	}								\
+	(typeof(*ptr))VAL;						\
+})
+#endif
+
+#ifndef smp_cond_load_relaxed_timewait
+#define smp_cond_load_relaxed_timewait  __smp_cond_load_relaxed_spinwait
+#endif
+
 /*
  * pmem_wmb() ensures that all stores for which the modification
  * are written to persistent storage by preceding instructions have
-- 
2.43.5