linux-kernel - Re: [pchecks v2 2/2] percpu: Add preemption checks to __this

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Date:	Fri, 4 Oct 2013 15:27:15 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Tejun Heo <tj@...nel.org>, akpm@...uxfoundation.org,
	Steven Rostedt <srostedt@...hat.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [pchecks v2 2/2] percpu: Add preemption checks to __this_cpu
 ops

On Fri, 4 Oct 2013, Peter Zijlstra wrote:

> > -# define __this_cpu_read(pcp)	__pcpu_size_call_return(__this_cpu_read_, (pcp))
> > +# define __this_cpu_read(pcp) \
> > +	(__this_cpu_preempt_check(),__pcpu_size_call_return(__this_cpu_read_, (pcp)))
> >  #endif
>
> Would it not be move convenient to implement it in terms of the
> raw_this_cpu*() thingies? That way you're sure they actually do the same
> thing and there's only 1 site to change when changing the
> implementation.

The __this_cpu_read_xxx() are asm primitives provided by various arches.
__this_cpu_read() is currently not overriden by any arches. That is why
the approach here of replicating only the higher level for raw_cpu_ops
works. Renaming the __this_cpu_xxx primitives would be a significant
change.

> >  	if (!printk_ratelimit())
> >  		goto out_enable;
> >
> > -	printk(KERN_ERR "BUG: using smp_processor_id() in preemptible [%08x] "
> > -			"code: %s/%d\n",
> > +	printk(KERN_ERR "%s in preemptible [%08x] "
> > +			"code: %s/%d\n", what,
> >  			preempt_count() - 1, current->comm, current->pid);
>
> I would argue for keeping the "BUG" string intact and in front of the
> %s.

Most of the place that I have seen are not bugs but there was a
reason for the code to run a __this_cpu op without preemption disabled.
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