Re: [PATCH v4 3/3] ring_buffer: Use try_cmpxchg instead of cmpxchg

[Mukesh Ojha]

On 3/5/2023 9:25 PM, Uros Bizjak 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.
>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> ---
> v2: Convert only loops with cmpxchg.
> ---
>   kernel/trace/ring_buffer.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 5235037f83d3..d17345b522f4 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -4061,10 +4061,10 @@ 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));
>   }
>   EXPORT_SYMBOL_GPL(ring_buffer_record_off);
>   
> @@ -4084,10 +4084,10 @@ 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));
>   }

Acked-by: Mukesh Ojha <quic_mojha@xxxxxxxxxxx>

-Mukesh
>   EXPORT_SYMBOL_GPL(ring_buffer_record_on);
>