linux-kernel - Re: [RFC][PATCH 5/7] irq_work, smp: Allow irq_work on call_single

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Date:   Fri, 5 Jun 2020 17:02:08 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org, x86@...nel.org,
        cai@....pw, mgorman@...hsingularity.net, sfr@...b.auug.org.au,
        linux@...ck-us.net
Subject: Re: [RFC][PATCH 5/7] irq_work, smp: Allow irq_work on
 call_single_queue

On Fri, Jun 05, 2020 at 11:37:04AM +0200, Peter Zijlstra wrote:
> On Fri, May 29, 2020 at 03:36:41PM +0200, Peter Zijlstra wrote:
> > Maybe I can anonymous-union my way around it, dunno. I'll think about
> > it. I'm certainly not proud of this. But at least the BUILD_BUG_ON()s
> > should catch the more blatant breakage here.
> 
> How's this then? Differently ugly, but at least it compiles with that
> horrible struct randomization junk enabled.
> 
> ---
>  include/linux/irq_work.h  |   28 ++++++-------------
>  include/linux/sched.h     |    4 +-
>  include/linux/smp.h       |   25 ++++++-----------
>  include/linux/smp_types.h |   66 ++++++++++++++++++++++++++++++++++++++++++++++
>  kernel/sched/core.c       |    6 ++--
>  kernel/smp.c              |   18 ------------
>  6 files changed, 89 insertions(+), 58 deletions(-)
> 
> --- a/include/linux/irq_work.h
> +++ b/include/linux/irq_work.h
> @@ -2,7 +2,7 @@
>  #ifndef _LINUX_IRQ_WORK_H
>  #define _LINUX_IRQ_WORK_H
>  
> -#include <linux/llist.h>
> +#include <linux/smp_types.h>
>  
>  /*
>   * An entry can be in one of four states:
> @@ -13,26 +13,16 @@
>   * busy      NULL, 2 -> {free, claimed} : callback in progress, can be claimed
>   */
>  
> -/* flags share CSD_FLAG_ space */
> -
> -#define IRQ_WORK_PENDING	BIT(0)
> -#define IRQ_WORK_BUSY		BIT(1)
> -
> -/* Doesn't want IPI, wait for tick: */
> -#define IRQ_WORK_LAZY		BIT(2)
> -/* Run hard IRQ context, even on RT */
> -#define IRQ_WORK_HARD_IRQ	BIT(3)
> -
> -#define IRQ_WORK_CLAIMED	(IRQ_WORK_PENDING | IRQ_WORK_BUSY)
> -
> -/*
> - * structure shares layout with single_call_data_t.
> - */
>  struct irq_work {
> -	struct llist_node llnode;
> -	atomic_t flags;
> +	union {
> +		struct __call_single_node node;
> +		struct {
> +			struct llist_node llnode;
> +			atomic_t flags;
> +		};
> +	};

So why not just embed struct __call_single_node in
struct irq_work and struct __call_single_data ?

Is the point of that anonymous second union layer to
shorten the lines while accessing members?

Thanks.