linux-kernel - Re: [gcv v3 06/35] scheduler: Replace __get_cpu

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Date:	Thu, 29 Aug 2013 09:58:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Tejun Heo <tj@...nel.org>, akpm@...uxfoundation.org,
	Ingo Molnar <mingo@...hat.com>, linux-arch@...r.kernel.org,
	Steven Rostedt <srostedt@...hat.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [gcv v3 06/35] scheduler: Replace __get_cpu_var uses

On Wed, Aug 28, 2013 at 07:48:14PM +0000, Christoph Lameter wrote:
> Transformations done to __get_cpu_var()
> 
> 
> 1. Determine the address of the percpu instance of the current processor.
> 
> 	DEFINE_PER_CPU(int, y);
> 	int *x = &__get_cpu_var(y);
> 
>     Converts to
> 
> 	int *x = this_cpu_ptr(&y);
> 
> 
> 2. Same as #1 but this time an array structure is involved.
> 
> 	DEFINE_PER_CPU(int, y[20]);
> 	int *x = __get_cpu_var(y);
> 
>     Converts to
> 
> 	int *x = this_cpu_ptr(y);
> 
> 
> 3. Retrieve the content of the current processors instance of a per cpu variable.
> 
> 	DEFINE_PER_CPU(int, u);
> 	int x = __get_cpu_var(y)
> 
>    Converts to
> 
> 	int x = __this_cpu_read(y);
> 

This looses a preemption debug check, so NAK

> 4. Retrieve the content of a percpu struct
> 
> 	DEFINE_PER_CPU(struct mystruct, y);
> 	struct mystruct x = __get_cpu_var(y);
> 
>    Converts to
> 
> 	memcpy(this_cpu_ptr(&x), y, sizeof(x));
> 
> 5. Assignment to a per cpu variable
> 
> 	DEFINE_PER_CPU(int, y)
> 	__get_cpu_var(y) = x;
> 
>    Converts to
> 
> 	this_cpu_write(y, x);
> 

This too looses a preemption debug check, NAK

> 6. Increment/Decrement etc of a per cpu variable
> 
> 	DEFINE_PER_CPU(int, y);
> 	__get_cpu_var(y)++
> 
>    Converts to
> 
> 	this_cpu_inc(y)
> 

Lo and behold.. no preemption checks again.


Seriously first fix the debug and validation bits of the *this_cpu*
stuff.
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