Will two READ_ONCE()s in a row execute in order

FMDF fmdefrancesco at gmail.com
Mon Oct 25 07:33:15 EDT 2021

On Mon, Oct 25, 2021 at 10:33 AM Zhang Zeren <zhangzr23 at gmail.com> wrote:
> Hi, I read an example in the Documentation/memory-barriers.txt, which says
>>  (*) On any given CPU, dependent memory accesses will be issued in order, with
>>      respect to itself.  This means that for:

"dependent" _is_ the key to understanding this topic. As Valentin
Vidić wrote, your loads are _not_ dependent and / or related in any

> As far as  I understand it, linux kernel memory model (LKMM) guarantee two read operations
> execute in order. And if the CPU architecture offer an looser memory ordering (like Alpha), then
> the compiler must help to add a memory barrier after the load instruction to fufill the LKMM's
> standard.

No, adding a memory barrier is not a compiler job, instead it is up to
the kernel code.
For instance, please take a look at the implementation of
__READ_ONCE() for Alpha in arch/alpha/include/asm/rwonce.h (note that
__READ_ONCE() is used by READ_ONCE()).


Fabio M. De Francesco

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