linux-kernel - [PATCH] READ_ONCE: Fix comment describing 2x32-bit atomicity

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Message-Id: <20200512090101.2497-1-will@kernel.org>
Date:   Tue, 12 May 2020 10:01:01 +0100
From:   Will Deacon <will@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>
Subject: [PATCH] READ_ONCE: Fix comment describing 2x32-bit atomicity

READ_ONCE() permits 64-bit accesses on 32-bit architectures, since this
crops up in a few places and is generally harmless because either the
upper bits are always zero (e.g. for a virtual address or 32-bit time_t)
or the architecture provides 64-bit atomicity anyway.

Update the corresponding comment above compiletime_assert_rwonce_type(),
which incorrectly states that 32-bit x86 provides 64-bit atomicity, and
instead reference 32-bit Armv7 with LPAE.

Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Reported-by: Jann Horn <jannh@...gle.com>
Signed-off-by: Will Deacon <will@...nel.org>
---

Applies on top of the READ_ONCE() pile I sent last night (v5).

 include/linux/compiler.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 741c93c62ecf..e24cc3a2bc3e 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -384,9 +384,9 @@ static inline void *offset_to_ptr(const int *off)
 
 /*
  * Yes, this permits 64-bit accesses on 32-bit architectures. These will
- * actually be atomic in many cases (namely x86), but for others we rely on
- * the access being split into 2x32-bit accesses for a 32-bit quantity (e.g.
- * a virtual address) and a strong prevailing wind.
+ * actually be atomic in some cases (namely Armv7 + LPAE), but for others we
+ * rely on the access being split into 2x32-bit accesses for a 32-bit quantity
+ * (e.g. a virtual address) and a strong prevailing wind.
  */
 #define compiletime_assert_rwonce_type(t)					\
 	compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long),	\
-- 
2.26.2.645.ge9eca65c58-goog