Re: [PATCH 3/3] ring_buffer: Use try_cmpxchg instead of cmpxchg

[Steven Rostedt]

On Tue, 28 Feb 2023 18:59:29 +0100
Uros Bizjak <ubizjak@xxxxxxxxx> wrote:

> Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old.
> x86 CMPXCHG instruction returns success in ZF flag, so this change
> saves a compare after cmpxchg (and related move instruction in
> front of cmpxchg).
> 
> Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
> fails. There is no need to re-read the value in the loop.
> 
> No functional change intended.

As I mentioned in the RCU thread, I have issues with some of the changes
here.

> 
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> ---
>  kernel/trace/ring_buffer.c | 20 ++++++++------------
>  1 file changed, 8 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 4188af7d4cfe..8f0ef7d12ddd 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -1493,14 +1493,11 @@ static bool rb_head_page_replace(struct buffer_page *old,
>  {
>  	unsigned long *ptr = (unsigned long *)&old->list.prev->next;
>  	unsigned long val;
> -	unsigned long ret;
>  
>  	val = *ptr & ~RB_FLAG_MASK;
>  	val |= RB_PAGE_HEAD;
>  
> -	ret = cmpxchg(ptr, val, (unsigned long)&new->list);
> -
> -	return ret == val;
> +	return try_cmpxchg(ptr, &val, (unsigned long)&new->list);

No, val should not be updated.

>  }
>  
>  /*
> @@ -2055,7 +2052,7 @@ rb_insert_pages(struct ring_buffer_per_cpu *cpu_buffer)
>  	retries = 10;
>  	success = false;
>  	while (retries--) {
> -		struct list_head *head_page, *prev_page, *r;
> +		struct list_head *head_page, *prev_page;
>  		struct list_head *last_page, *first_page;
>  		struct list_head *head_page_with_bit;
>  
> @@ -2073,9 +2070,8 @@ rb_insert_pages(struct ring_buffer_per_cpu *cpu_buffer)
>  		last_page->next = head_page_with_bit;
>  		first_page->prev = prev_page;
>  
> -		r = cmpxchg(&prev_page->next, head_page_with_bit, first_page);
> -
> -		if (r == head_page_with_bit) {
> +		if (try_cmpxchg(&prev_page->next,
> +				&head_page_with_bit, first_page)) {

No. head_page_with_bit should not be updated.

>  			/*
>  			 * yay, we replaced the page pointer to our new list,
>  			 * now, we just have to update to head page's prev
> @@ -4061,10 +4057,10 @@ void ring_buffer_record_off(struct trace_buffer *buffer)
>  	unsigned int rd;
>  	unsigned int new_rd;
>  
> +	rd = atomic_read(&buffer->record_disabled);
>  	do {
> -		rd = atomic_read(&buffer->record_disabled);
>  		new_rd = rd | RB_BUFFER_OFF;
> -	} while (atomic_cmpxchg(&buffer->record_disabled, rd, new_rd) != rd);
> +	} while (!atomic_try_cmpxchg(&buffer->record_disabled, &rd, new_rd));

This change is OK.

>  }
>  EXPORT_SYMBOL_GPL(ring_buffer_record_off);
>  
> @@ -4084,10 +4080,10 @@ void ring_buffer_record_on(struct trace_buffer *buffer)
>  	unsigned int rd;
>  	unsigned int new_rd;
>  
> +	rd = atomic_read(&buffer->record_disabled);
>  	do {
> -		rd = atomic_read(&buffer->record_disabled);
>  		new_rd = rd & ~RB_BUFFER_OFF;
> -	} while (atomic_cmpxchg(&buffer->record_disabled, rd, new_rd) != rd);
> +	} while (!atomic_try_cmpxchg(&buffer->record_disabled, &rd, new_rd));

And so is this one.

So I will not take this patch as is.

-- Steve

>  }
>  EXPORT_SYMBOL_GPL(ring_buffer_record_on);
>