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@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index c363d8debc43fa8405fee32a93c2029c8b2303e0..657e4fd38a7718024c7292de22751ad5e3abcc26 100644 (file)
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -332,9 +332,9 @@ 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),  \