Re: [PATCH] rcu: use try_cmpxchg in check_cpu_stall

[Steven Rostedt]

On Wed, 1 Mar 2023 20:18:11 +0100
Uros Bizjak <ubizjak@xxxxxxxxx> wrote:

> If we want to put the definition in generic headers, then we also need
> to define acquire/release/relaxed and 64bit variants. ATM, we have two
> sites that require this definition and I think that for now we could
> live with two instances of the same definition in two separate
> subsystems. But this would definitely be a good future addition. There
> is some code in the form of
> 
> if (cmpxchg (ptr, 0, 1) == 0)
> 
> that can not be converted to try_cmpxchg, but can use cmpxchg_success.

And even modify code that uses temp variables. For example, where you
modified the ring buffer code to use try_cmpxchg(), I could convert your:

static int rb_head_page_replace(struct buffer_page *old,
				struct buffer_page *new)
{
	unsigned long *ptr = (unsigned long *)&old->list.prev->next;
	unsigned long val;

	val = *ptr & ~RB_FLAG_MASK;
	val |= RB_PAGE_HEAD;

	return try_cmpxchg(ptr, &val, (unsigned long)&new->list);
}

Into just:

static int rb_head_page_replace(struct buffer_page *old,
				struct buffer_page *new)
{
	unsigned long *ptr = (unsigned long *)&old->list.prev->next;
	unsigned long val;

	val = *ptr & ~RB_FLAG_MASK;

	return cmpxchg_success(ptr, val | RB_PAGE_HEAD, (unsigned long)&new->list);
}

-- Steve