linux-kernel - [PATCH v4 2/5] arm64: barrier: Add smp_cond_load_relaxed

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Message-Id: <20250829080735.3598416-3-ankur.a.arora@oracle.com>
Date: Fri, 29 Aug 2025 01:07:32 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, bpf@...r.kernel.org
Cc: arnd@...db.de, catalin.marinas@....com, will@...nel.org,
        peterz@...radead.org, akpm@...ux-foundation.org, mark.rutland@....com,
        harisokn@...zon.com, cl@...two.org, ast@...nel.org, memxor@...il.com,
        zhenglifeng1@...wei.com, xueshuai@...ux.alibaba.com,
        joao.m.martins@...cle.com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com
Subject: [PATCH v4 2/5] arm64: barrier: Add smp_cond_load_relaxed_timewait()

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
spin-waiting.

Cc: Will Deacon <will@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org
Suggested-by: Catalin Marinas <catalin.marinas@....com>
Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
---
 arch/arm64/include/asm/barrier.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index f5801b0ba9e9..9b29abc212db 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -219,6 +219,28 @@ do {									\
 	(typeof(*ptr))VAL;						\
 })
 
+extern bool arch_timer_evtstrm_available(void);
+
+#define smp_cond_load_relaxed_timewait(ptr, cond_expr, time_check_expr)	\
+({									\
+	typeof(ptr) __PTR = (ptr);					\
+	__unqual_scalar_typeof(*ptr) VAL;				\
+	bool __wfe = arch_timer_evtstrm_available();			\
+									\
+	for (;;) {							\
+		VAL = READ_ONCE(*__PTR);				\
+		if (cond_expr)						\
+			break;						\
+		if (time_check_expr)					\
+			break;						\
+		if (likely(__wfe))					\
+			__cmpwait_relaxed(__PTR, VAL);			\
+		else							\
+			cpu_relax();					\
+	}								\
+	(typeof(*ptr)) VAL;						\
+})
+
 #include <asm-generic/barrier.h>
 
 #endif	/* __ASSEMBLY__ */
-- 
2.31.1