linux-kernel - [PATCH v4 3/4] arm64: alternatives: Remove READ

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Date:   Tue,  3 Nov 2020 12:17:20 +0000
From:   Will Deacon <will@...nel.org>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     Will Deacon <will@...nel.org>, Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v4 3/4] arm64: alternatives: Remove READ_ONCE() usage during patch operation

In preparation for patching the internals of READ_ONCE() itself, replace
its usage on the alternatives patching patch with a volatile variable
instead.

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Will Deacon <will@...nel.org>
---
 arch/arm64/kernel/alternative.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
index 73039949b5ce..a57cffb752e8 100644
--- a/arch/arm64/kernel/alternative.c
+++ b/arch/arm64/kernel/alternative.c
@@ -21,7 +21,8 @@
 #define ALT_ORIG_PTR(a)		__ALT_PTR(a, orig_offset)
 #define ALT_REPL_PTR(a)		__ALT_PTR(a, alt_offset)
 
-static int all_alternatives_applied;
+/* Volatile, as we may be patching the guts of READ_ONCE() */
+static volatile int all_alternatives_applied;
 
 static DECLARE_BITMAP(applied_alternatives, ARM64_NCAPS);
 
@@ -205,7 +206,7 @@ static int __apply_alternatives_multi_stop(void *unused)
 
 	/* We always have a CPU 0 at this point (__init) */
 	if (smp_processor_id()) {
-		while (!READ_ONCE(all_alternatives_applied))
+		while (!all_alternatives_applied)
 			cpu_relax();
 		isb();
 	} else {
@@ -217,7 +218,7 @@ static int __apply_alternatives_multi_stop(void *unused)
 		BUG_ON(all_alternatives_applied);
 		__apply_alternatives(&region, false, remaining_capabilities);
 		/* Barriers provided by the cache flushing */
-		WRITE_ONCE(all_alternatives_applied, 1);
+		all_alternatives_applied = 1;
 	}
 
 	return 0;
-- 
2.29.1.341.ge80a0c044ae-goog