linux-kernel - Re: [PATCH v5 03/15] sched/core: uclamp: map TASK's clamp values into CPU's clamp groups

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Date:   Wed, 7 Nov 2018 13:57:38 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v5 03/15] sched/core: uclamp: map TASK's clamp values
 into CPU's clamp groups

On 07-Nov 14:16, Peter Zijlstra wrote:
> On Mon, Oct 29, 2018 at 06:32:57PM +0000, Patrick Bellasi wrote:
> 
> > +static void uclamp_group_put(unsigned int clamp_id, unsigned int group_id)
> >  {
> > +	union uclamp_map *uc_maps = &uclamp_maps[clamp_id][0];
> > +	union uclamp_map uc_map_old, uc_map_new;
> > +	long res;
> > +
> > +retry:
> > +
> > +	uc_map_old.data = atomic_long_read(&uc_maps[group_id].adata);
> > +	uc_map_new = uc_map_old;
> > +	uc_map_new.se_count -= 1;
> > +	res = atomic_long_cmpxchg(&uc_maps[group_id].adata,
> > +				  uc_map_old.data, uc_map_new.data);
> > +	if (res != uc_map_old.data)
> > +		goto retry;
> > +}
> 
> Please write cmpxchg loops in the form:
> 
> 	atomic_long_t *ptr = &uclamp_maps[clamp_id][group_id].adata;
> 	union uclamp_map old, new;
> 
> 	old.data = atomic_long_read(ptr);
> 	do {
> 		new.data = old.data;
> 		new.se_cound--;
> 	} while (!atomic_long_try_cmpxchg(ptr, &old.data, new.data));
> 
> 
> (same for all the others of course)

Ok, I did that to save some indentation, but actually it's most
commonly used in a while loop... will update in v6.

Out of curiosity, apart from code consistency, is that required also
specifically for any possible compiler related (mis)behavior ?

-- 
#include <best/regards.h>

Patrick Bellasi