linux-kernel - Re: sched: current instead rq->current

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Date:	Thu, 18 Aug 2016 13:28:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Colin Vidal <colin@...dal.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com
Subject: Re: sched: current instead rq->current

On Wed, Aug 17, 2016 at 09:39:41PM +0200, Colin Vidal wrote:
> On Mon, 2016-08-15 at 21:30 +0200, Colin Vidal wrote:
> > Hello,
> > 
> > At the beginning of __schedule (kernel/sched/core.c), the current
> > task
> > is get with rq->curr. I try to to understand why not directly using
> > current instead?

> > -       int cpu;
> >  
> > -       cpu = smp_processor_id();
> > -       rq = cpu_rq(cpu);
> > -       prev = rq->curr;
> > +       rq = cpu_rq(smp_processor_id());
> > +       prev = current;
> > 
> > and it seems to work (only tested on x86-64), but... To simple?

its more expensive, rq->curr is a simple dereference (and we need that
cacheline anyway), while current is a weird macro that expands to
potentially a lot of code, depending on the arch.