[PATCH v9 01/12] asm-generic: barrier: Add smp_cond_load_relaxed_timeout()

[Ankur Arora <ankur.a.arora@oracle.com>]

Add smp_cond_load_relaxed_timeout(), which extends
smp_cond_load_relaxed() to allow waiting for a duration.

We loop around waiting for the condition variable to change while
peridically doing a time-check. The loop uses cpu_poll_relax() to slow
down the busy-waiting, which, unless overridden by the architecture
code, amounts to a cpu_relax().

Note that there are two ways for the time-check to fail: the usual
timeout case or, @time_expr_ns returning an invalid value (negative
or zero). The second failure mode allows for clocks attached to the
clock-domain of @cond_expr, which might cease to operate meaningfully
once some state internal to @cond_expr has changed.

Evaluation of @time_expr_ns: in the fastpath we want to keep the
performance close to smp_cond_load_relaxed(). To do that we defer
evaluation of the potentially costly @time_expr_ns to when we hit
the slowpath.

This also means that there will always be some hardware dependent
duration that has passed in cpu_poll_relax() iterations at the time of
first evaluation. Additionally cpu_poll_relax() is not guaranteed to
return at timeout boundary. In sum, expect timeout overshoot when we
exit due to expiration of the timeout.

The number of spin iterations before time-check, SMP_TIMEOUT_POLL_COUNT
is chosen to be 200 by default. With a cpu_poll_relax() iteration
taking ~20-30 cycles (measured on a variety of x86 platforms), we expect
a tim-check every ~4000-6000 cycles.

The outer limit of the overshoot is double that when working with the
parameters above. This might be higher or lower depending on the
implementation of cpu_poll_relax() across architectures.

Lastly, config option ARCH_HAS_CPU_RELAX indicates availability of a
cpu_poll_relax() that is cheaper than polling. This might be relevant
for cases with a prolonged timeout.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-arch@vger.kernel.org
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
Notes:
  - Defer evaluation of @time_expr_ns to when we hit the slowpath.
  - This also helps get rid of the labelled gotos which were used to
    handle the early failure case (since now there's no early init
    to be concerned with.)
  - Add a comment mentioning that the cpu_poll_relax() implementation
    is better than polling if ARCH_HAS_CPU_RELAX.

 include/asm-generic/barrier.h | 72 +++++++++++++++++++++++++++++++++++
 1 file changed, 72 insertions(+)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index d4f581c1e21d..2738fe35c1df 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -273,6 +273,68 @@ do {									\
 })
 #endif
 
+/*
+ * Number of times we iterate in the loop before doing the time check.
+ */
+#ifndef SMP_TIMEOUT_POLL_COUNT
+#define SMP_TIMEOUT_POLL_COUNT		200
+#endif
+
+/*
+ * Platforms with ARCH_HAS_CPU_RELAX have a cpu_poll_relax() implementation
+ * that is expected to be cheaper (lower power) than pure polling.
+ */
+#ifndef cpu_poll_relax
+#define cpu_poll_relax(ptr, val, timeout_ns)	cpu_relax()
+#endif
+
+/**
+ * smp_cond_load_relaxed_timeout() - (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: expression that evaluates to monotonic time (in ns) or,
+ *  on failure, returns a negative value.
+ * @timeout_ns: timeout value in ns
+ * Both of the above are assumed to be compatible with s64; the signed
+ * value is used to handle the failure case in @time_expr_ns.
+ *
+ * Equivalent to using READ_ONCE() on the condition variable.
+ *
+ * Callers that expect to wait for prolonged durations might want to
+ * take into account the availability of ARCH_HAS_CPU_RELAX.
+ */
+#ifndef smp_cond_load_relaxed_timeout
+#define smp_cond_load_relaxed_timeout(ptr, cond_expr,			\
+				      time_expr_ns, timeout_ns)		\
+({									\
+	typeof(ptr) __PTR = (ptr);					\
+	__unqual_scalar_typeof(*ptr) VAL;				\
+	u32 __n = 0, __spin = SMP_TIMEOUT_POLL_COUNT;			\
+	s64 __timeout = (s64)timeout_ns;				\
+	s64 __time_now, __time_end = 0;					\
+									\
+	for (;;) {							\
+		VAL = READ_ONCE(*__PTR);				\
+		if (cond_expr) 						\
+			break;						\
+		cpu_poll_relax(__PTR, VAL, (u64)__timeout);		\
+		if (++__n < __spin)					\
+			continue;					\
+		__time_now = (s64)(time_expr_ns);			\
+		if (unlikely(__time_end == 0))				\
+			__time_end = __time_now + __timeout;		\
+		__timeout = __time_end - __time_now;			\
+		if (__time_now <= 0 || __timeout <= 0) {		\
+			VAL = READ_ONCE(*__PTR);			\
+			break;						\
+		}							\
+		__n = 0;						\
+	}								\
+	(typeof(*ptr))VAL;						\
+})
+#endif
+
 /*
  * pmem_wmb() ensures that all stores for which the modification
  * are written to persistent storage by preceding instructions have
-- 
2.31.1