[PATCH 3/4] arm64: barrier: Add smp_cond_load_relaxed_timewait()

Ankur Arora
2025-02-03T21:49:43Z

Add smp_cond_load_relaxed_timewait(), a timed variant of
smp_cond_load_relaxed().

This uses __cmpwait_relaxed() to do the actual waiting, with the
event-stream guaranteeing that we wake up from WFE periodically
and not block forever in case there are no stores to the cacheline.

For cases when the event-stream is unavailable, fallback to the
generic spin-wait implementation.

Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
arch/arm64/include/asm/barrier.h | 38 ++++++++++++++++++++++++++++++++
1 file changed, 38 insertions(+)

diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index 1ca947d5c939..25721275a5a2 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -216,6 +216,44 @@ do { \
(typeof(*ptr))VAL; \
})

+#define __smp_cond_load_relaxed_timewait(ptr, cond_expr, \
+ time_expr_ns, time_limit_ns) \
+({ \
+ typeof(ptr) __PTR = (ptr); \
+ __unqual_scalar_typeof(*ptr) VAL; \
+ for (;;) { \
+ VAL = READ_ONCE(*__PTR); \
+ if (cond_expr) \
+ break; \
+ __cmpwait_relaxed(__PTR, VAL); \
+ if ((time_expr_ns) >= (time_limit_ns)) \
+ break; \
+ } \
+ (typeof(*ptr))VAL; \
+})
+
+/*
+ * For the unlikely case that the event-stream is unavailable,
+ * ward off the possibility of waiting forever by falling back
+ * to the generic spin-wait.
+ */
+#define smp_cond_load_relaxed_timewait(ptr, cond_expr, \
+ time_expr_ns, time_limit_ns) \
+({ \
+ __unqual_scalar_typeof(*ptr) _val; \
+ int __wfe = arch_timer_evtstrm_available(); \
+ \
+ if (likely(__wfe)) \
+ _val = __smp_cond_load_relaxed_timewait(ptr, cond_expr, \
+ time_expr_ns, \
+ time_limit_ns); \
+ else \
+ _val = __smp_cond_load_relaxed_spinwait(ptr, cond_expr, \
+ time_expr_ns, \
+ time_limit_ns); \
+ (typeof(*ptr))_val; \
+})
+
#include <asm-generic/barrier.h>

#endif /* __ASSEMBLY__ */
--
2.43.5