linux-kernel - Re: [PATCH -mm 1/2] irq_work, Use llist in irq

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Message-ID: <1314863927.7945.11.camel@twins>
Date:	Thu, 01 Sep 2011 09:58:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Huang Ying <ying.huang@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm 1/2] irq_work, Use llist in irq_work

On Thu, 2011-09-01 at 11:20 +0800, Huang Ying wrote:
> On 09/01/2011 09:46 AM, Huang Ying wrote:
> >>> -static void __irq_work_queue(struct irq_work *entry)
> >>> +static void __irq_work_queue(struct irq_work *work)
> >>>  {
> >>> -	struct irq_work *next;
> >>> +	struct irq_work_list *irq_work_list;
> >>>  
> >>> -	preempt_disable();
> >>> +	irq_work_list = &get_cpu_var(irq_work_lists);
> >>>  
> >>> -	do {
> >>> -		next = __this_cpu_read(irq_work_list);
> >>> -		/* Can assign non-atomic because we keep the flags set. */
> >>> -		entry->next = next_flags(next, IRQ_WORK_FLAGS);
> >>> -	} while (this_cpu_cmpxchg(irq_work_list, next, entry) != next);
> >>> +	llist_add(&work->llnode, &irq_work_list->llist);
> >>>  
> >>>  	/* The list was empty, raise self-interrupt to start processing. */
> >>> -	if (!irq_work_next(entry))
> >>> +	if (!test_and_set_bit(LIST_NONEMPTY_BIT, &irq_work_list->flags))
> >>>  		arch_irq_work_raise();
> >>
> >> So why can't you simply test work->llnode->next?
> > 
> > Yes.  That is better.  Even if there may be a small race window, it is
> > not a big issue to raise one extra self interrupt seldom.
> 
> Remember something about this.  I didn't test work->llnode->next here
> because I didn't want expose the implementation details like that here.
>  How about make llist_add() return whether list is empty before adding?
>  Because it will be an inline function, that should be optimized out if
> the caller do not need the information.

You could also use llist_empty() although that brings me to that
ACCESS_ONCE thing in there, what's the point?
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