linux-kernel - Re: [PATCH] ring-buffer: Simplify ring_buffer_read

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Message-Id: <20250528093116.2451fe1b1ebad2f457594c57@kernel.org>
Date: Wed, 28 May 2025 09:31:16 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] ring-buffer: Simplify ring_buffer_read_page() with
 guard()

On Tue, 27 May 2025 14:52:16 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: Steven Rostedt <rostedt@...dmis.org>
> 
> The function ring_buffer_read_page() had two gotos. One was simply
> returning "ret" and the other was unlocking the reader_lock.
> 
> There's no reason to use goto to simply return the "ret" variable. Instead
> just return the value.
> 
> The jump to the unlocking of the reader_lock can be replaced by
> guard(raw_spinlock_irqsave)(&cpu_buffer->reader_lock).
> 
> With these two changes the "ret" variable is no longer used and can be
> removed. The return value on non-error is what was read and is stored in
> the "read" variable.
> 

Looks good to me.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thanks!

> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  kernel/trace/ring_buffer.c | 28 +++++++++++-----------------
>  1 file changed, 11 insertions(+), 17 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 0b21d64e90c8..897ce51d3bbf 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -6535,38 +6535,37 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
>  	struct buffer_data_page *bpage;
>  	struct buffer_page *reader;
>  	unsigned long missed_events;
> -	unsigned long flags;
>  	unsigned int commit;
>  	unsigned int read;
>  	u64 save_timestamp;
> -	int ret = -1;
>  
>  	if (!cpumask_test_cpu(cpu, buffer->cpumask))
> -		goto out;
> +		return -1;
>  
>  	/*
>  	 * If len is not big enough to hold the page header, then
>  	 * we can not copy anything.
>  	 */
>  	if (len <= BUF_PAGE_HDR_SIZE)
> -		goto out;
> +		return -1;
>  
>  	len -= BUF_PAGE_HDR_SIZE;
>  
>  	if (!data_page || !data_page->data)
> -		goto out;
> +		return -1;
> +
>  	if (data_page->order != buffer->subbuf_order)
> -		goto out;
> +		return -1;
>  
>  	bpage = data_page->data;
>  	if (!bpage)
> -		goto out;
> +		return -1;
>  
> -	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
> +	guard(raw_spinlock_irqsave)(&cpu_buffer->reader_lock);
>  
>  	reader = rb_get_reader_page(cpu_buffer);
>  	if (!reader)
> -		goto out_unlock;
> +		return -1;
>  
>  	event = rb_reader_event(cpu_buffer);
>  
> @@ -6600,7 +6599,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
>  		if (full &&
>  		    (!read || (len < (commit - read)) ||
>  		     cpu_buffer->reader_page == cpu_buffer->commit_page))
> -			goto out_unlock;
> +			return -1;
>  
>  		if (len > (commit - read))
>  			len = (commit - read);
> @@ -6609,7 +6608,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
>  		size = rb_event_ts_length(event);
>  
>  		if (len < size)
> -			goto out_unlock;
> +			return -1;
>  
>  		/* save the current timestamp, since the user will need it */
>  		save_timestamp = cpu_buffer->read_stamp;
> @@ -6667,7 +6666,6 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
>  		if (reader->real_end)
>  			local_set(&bpage->commit, reader->real_end);
>  	}
> -	ret = read;
>  
>  	cpu_buffer->lost_events = 0;
>  
> @@ -6694,11 +6692,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
>  	if (commit < buffer->subbuf_size)
>  		memset(&bpage->data[commit], 0, buffer->subbuf_size - commit);
>  
> - out_unlock:
> -	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
> -
> - out:
> -	return ret;
> +	return read;
>  }
>  EXPORT_SYMBOL_GPL(ring_buffer_read_page);
>  
> -- 
> 2.47.2
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>