linux-kernel - Re: [RFC][PATCH 2/2] sched: Use fancy new guards

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Date:   Fri, 26 May 2023 10:08:39 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        torvalds@...ux-foundation.org, pbonzini@...hat.com,
        linux-kernel@...r.kernel.org, ojeda@...nel.org,
        ndesaulniers@...gle.com, mingo@...hat.com, will@...nel.org,
        longman@...hat.com, boqun.feng@...il.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, paulmck@...nel.org,
        frederic@...nel.org, quic_neeraju@...cinc.com,
        joel@...lfernandes.org, josh@...htriplett.org,
        jiangshanlai@...il.com, qiang1.zhang@...el.com,
        rcu@...r.kernel.org, tj@...nel.org, tglx@...utronix.de
Subject: Re: [RFC][PATCH 2/2] sched: Use fancy new guards

On Fri, May 26, 2023 at 12:27:51PM -0400, Mathieu Desnoyers wrote:
> On 5/26/23 12:25, Greg KH wrote:
> > On Fri, May 26, 2023 at 05:05:51PM +0200, Peter Zijlstra wrote:
> > > Convert kernel/sched/core.c to use the fancy new guards to simplify
> > > the error paths.
> > 
> > That's slightly crazy...
> > 
> > I like the idea, but is this really correct:
> > 
> > 
> > > 
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > > ---
> > >   kernel/sched/core.c  | 1223 +++++++++++++++++++++++----------------------------
> > >   kernel/sched/sched.h |   39 +
> > >   2 files changed, 595 insertions(+), 667 deletions(-)
> > > 
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -1097,24 +1097,21 @@ int get_nohz_timer_target(void)
> > >   	hk_mask = housekeeping_cpumask(HK_TYPE_TIMER);
> > > -	rcu_read_lock();
> > > -	for_each_domain(cpu, sd) {
> > > -		for_each_cpu_and(i, sched_domain_span(sd), hk_mask) {
> > > -			if (cpu == i)
> > > -				continue;
> > > +	void_scope(rcu) {
> > > +		for_each_domain(cpu, sd) {
> > > +			for_each_cpu_and(i, sched_domain_span(sd), hk_mask) {
> > > +				if (cpu == i)
> > > +					continue;
> > > -			if (!idle_cpu(i)) {
> > > -				cpu = i;
> > > -				goto unlock;
> > > +				if (!idle_cpu(i))
> > > +					return i;
> > 
> > You can call return from within a "scope" and it will clean up properly?
> > 
> > I tried to read the cpp "mess" but couldn't figure out how to validate
> > this at all, have a set of tests for this somewhere?
> > 
> > Anyway, the naming is whack, but I don't have a proposed better name,
> > except you might want to put "scope_" as the prefix not the suffix, but
> > then that might look odd to, so who knows.
> 
> FWIW C++ has std::scoped_lock. So perhaps using a similar wording may help ?

Yeah, I like "scoped_*" and "guarded_*" for naming. IMO, it reads better.

-- 
Kees Cook