linux-kernel - [PATCH v7 2/7] arm64: barrier: Support smp_cond_load_relaxed

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Message-Id: <20251017061606.455701-3-ankur.a.arora@oracle.com>
Date: Thu, 16 Oct 2025 23:16:01 -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, rafael@...nel.org,
        daniel.lezcano@...aro.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 v7 2/7] arm64: barrier: Support smp_cond_load_relaxed_timeout()

Support waiting in smp_cond_load_relaxed_timeout() via
__cmpwait_relaxed(). Limit this to when the event-stream is enabled,
to ensure that we wake from WFE periodically and don't block forever
if there are no stores to the cacheline.

In the unlikely event that the event-stream is unavailable, fallback
to spin-waiting.

Also set SMP_TIMEOUT_POLL_COUNT to 1 so we do the time-check for each
iteration in smp_cond_load_relaxed_timeout().

Cc: linux-arm-kernel@...ts.infradead.org
Cc: Catalin Marinas <catalin.marinas@....com>
Suggested-by: Will Deacon <will@...nel.org>
Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
---
 arch/arm64/include/asm/barrier.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index f5801b0ba9e9..92c16dfb8ca6 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -219,6 +219,19 @@ do {									\
 	(typeof(*ptr))VAL;						\
 })
 
+#define SMP_TIMEOUT_POLL_COUNT	1
+
+/* Re-declared here to avoid include dependency. */
+extern bool arch_timer_evtstrm_available(void);
+
+#define cpu_poll_relax(ptr, val)					\
+do {									\
+	if (arch_timer_evtstrm_available())				\
+		__cmpwait_relaxed(ptr, val);				\
+	else								\
+		cpu_relax();						\
+} while (0)
+
 #include <asm-generic/barrier.h>
 
 #endif	/* __ASSEMBLY__ */
-- 
2.43.5