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SubjectRe: [PATCH v4 3/3] ring_buffer: Use try_cmpxchg instead of cmpxchg
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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@goodmis.org>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> ---
> 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@quicinc.com>

-Mukesh
> EXPORT_SYMBOL_GPL(ring_buffer_record_on);
>

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