Re: [RFC][PATCH 5/7] irq_work, smp: Allow irq_work on call_single_queue

[Peter Zijlstra <peterz@infradead.org>]

On Fri, May 29, 2020 at 01:40:32AM +0200, Frederic Weisbecker wrote:
> On Tue, May 26, 2020 at 06:11:02PM +0200, Peter Zijlstra wrote:

> > +/*
> > + * structure shares layout with single_call_data_t.
> > + */
> >  struct irq_work {
> > -	atomic_t flags;
> >  	struct llist_node llnode;
> > +	atomic_t flags;
> 
> 
> We should probably have:
> 
> struct csd_node {
>        atomic_t flags;
>        struct llist_node;
> }
> 
> embed inside struct irq_work and struct __call_single_data. Relying on
> structure layout for things to work doesn't really clarify things :-)

Yes I know, but changing those structures is going to cause an aweful
lot of churn, and I didn't want to do that just now.. :-(

Also, there's more fun..

  CSD_TYPE_SYNC/ASYNC:

	struct {
		struct llist_node node;
		unsigned int flags;
		smp_call_func_t func;
		void *info;
	};

  CSD_TYPE_IRQ_WORK:

	struct {
		struct llist_node node;
		atomic_t flags;
		void (*func)(struct irq_work *);
	};

  CSD_TYPE_TTWU:

	struct {
		struct llist_node node;
		unsigned int flags;
	};

So while they all have a 'u32' sized @flags, irq_work wants it atomic.
Also, if we were to actually have the struct csd_node {}, you get a 4
byte hole when you embed it in task_struct.

This is all entirely fugly. No doubt about it.

But I failed to find a 'sane' way to express it and needed to get these
patches out because things were broken.

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.