On Fri, Mar 4, 2016 at 1:05 AM, Darren Hart <dvh...@infradead.org> wrote:
> I thought I provided a corrected comment block.... maybe I didn't. We have
> been
> working on improving the futex documentation, so we're paying close attention
> to
> terminology as well as grammar. This one needs a couple minor tweaks. I
> suggest:
>
> /*
> * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
> * optimizing lock_ptr out of the logic below.
> */
>
> The bit about q->lock_ptr possibly changing is already covered by the large
> comment block below the spin_lock(lock_ptr) call.
The large comment block is explaining the why the retry logic is required.
To achieve this semantic requirement, the READ_ONCE is needed to prevent
compiler optimizing it by doing double loads.
So I think the comment above should explain this tricky part.
> /* Use READ_ONCE to forbid the compiler from reloading q->lock_ptr in
> spin_lock() */
And as for preventing from optimizing the lock_ptr out of the retry
code block, I have consult
Paul Mckenney, he suggests one more READ_ONCE should be added here:
if (unlikely(lock_ptr != READ_ONCE(q->lock_ptr))) {
<------------------------------
spin_unlock(lock_ptr);
goto retry;
}
And I think this are two problem, and should be separated into two patches?
Regards,
Jianyu Zhan
