linux-kernel - Re: [PATCH] ring-buffer: Have the buffer update counter be atomic

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Message-ID: <1924e096-916a-4311-a3d5-07d3813f50da@suse.com>
Date: Fri, 11 Oct 2024 10:05:51 +0200
From: Petr Pavlu <petr.pavlu@...e.com>
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: Have the buffer update counter be atomic

On 10/11/24 01:58, Steven Rostedt wrote:
> From: Steven Rostedt <rostedt@...dmis.org>
> 
> In order to prevent any subtle races with the buffer update counter,
> change it to an atomic_t. Also, since atomic_t is 32 bits, move its
> location in the ring_buffer_per_cpu structure next to "current_context" as
> that too is only 32 bits (making 64 bit alignment).
> 
> The counter is only used to detect that the buffer has been updated when
> the buffer verifier check is being done. It's not really that important
> that it's atomic or not. But since the updates to the counter are never in
> the fast path, having it be consistent isn't a bad thing.
> 
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
> Note, this is based on top of:
> 
>   https://lore.kernel.org/linux-trace-kernel/20240715145141.5528-1-petr.pavlu@suse.com/

Sorry for not replying to your last comment on my patch, I was ill.

The member ring_buffer_per_cpu.cnt is intended to be accessed under the
reader_lock, same as the pages pointer which it is tied to, so this
change shouldn't be strictly needed.

-- 
Thanks,
Petr