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 <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Reported-by: Jann Horn <[email protected]>
Signed-off-by: Will Deacon <[email protected]>

Author: willdeacon

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),	\