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Subject: Re: per-cpu operation madness vs validation
From: Peter Zijlstra <peterz@infradead.org>
To: Christoph Lameter <cl@linux.com>
Cc: Tejun Heo <tj@kernel.org>, Linus Torvalds <torvalds@linux-foundation.org>,
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        Ingo Molnar <mingo@elte.hu>
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Date: Wed, 27 Jul 2011 15:01:17 +0200
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On Wed, 2011-07-27 at 14:11 +0200, Peter Zijlstra wrote:

> Hence my suggestion to do something like:
> 
> struct foo {
> 	percpu_lock_t lock;
> 	int a;
> 	int b;
> }
> 
> DEFINE_PER_CPU(struct foo, foo);
> 
> percpu_lock(&foo.lock);
> __this_cpu_inc(foo.a);
> __this_cpu_inc(foo.b);
> percpu_unlock(&foo.lock);
> 
> That would get us (aside from a shitload of work to make it so):
> 
>  - clear boundaries of where the data structure atomicy lie
>  - validation, for if the above piece of code was also ran from IRQ
>    context we could get lockdep complaining about IRQ unsafe locks used
>    from IRQ context.
> 
> Now for !-rt percpu_lock will not emit more than
> preempt_disable/local_bh_disable/local_irq_disable, depending on what
> variant is used, and the data type percpu_lock_t would be empty (except
> when enabling lockdep of course).
> 
> Possibly we could reduce all this percpu madness back to one form
> (__this_cpu_*) and require that when used a lock of the percpu_lock_t is
> taken.

get_cpu_var()/put_cpu_var() were supposed to provide such delineation as
well, but you've been actively destroying things like that with the
recent per-cpu work.

Also, I think we can mostly deprecate preempt_disable, local_bh_disable
and local_irq_disable when we have percpu_lock_t, or is local_lock_t a
better name?