Re: [tip: locking/urgent] locking/ww_mutex: Simplify use_ww_ctx & ww_ctx handling

From: Waiman Long
Date: Wed Mar 17 2021 - 10:10:57 EST

On 3/17/21 9:55 AM, Peter Zijlstra wrote:
On Wed, Mar 17, 2021 at 09:43:20AM -0400, Waiman Long wrote:

Using gcc 8.4.1, the generated __mutex_lock function has the same size (with
last instruction at offset +5179) with or without this patch. Well, you can
say that this patch is an no-op wrt generated code.
OK, then GCC has gotten better. Because back then I tried really hard
but it wouldn't remove the if (ww_ctx) branches unless I had that extra
const bool argument.

I think ww_mutex was merged in 2013. That is almost 8 years ago. It could still be the case that older gcc compilers may not generate the right code. I will try the RHEL7 gcc compiler (4.8.5) to see how it fares.