linux-kernel - Re: [PATCH] ring-buffer: use READ_ONCE() to read cpu_buffer->commit

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Date: Fri, 1 Mar 2024 10:49:45 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: linke <lilinke99@...com>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 mathieu.desnoyers@...icios.com, mhiramat@...nel.org
Subject: Re: [PATCH] ring-buffer: use READ_ONCE() to read
 cpu_buffer->commit_page in concurrent environment

On Fri, 1 Mar 2024 13:37:18 +0800
linke <lilinke99@...com> wrote:

> > So basically you are worried about read-tearing?
> > 
> > That wasn't mentioned in the change log.  
> 
> Yes. Sorry for making this confused, I am not very familiar with this and
> still learning.

No problem. We all have to learn this anyway.

> 
> > Funny part is, if the above timestamp read did a tear, then this would
> > definitely not match, and would return the correct value. That is, the
> > buffer is not empty because the only way for this to get corrupted is if
> > something is in the process of writing to it.  
> 
> I agree with you here.
> 
> 	commit = rb_page_commit(commit_page);
> 
> But if commit_page above is the result of a torn read, the commit field
> read by rb_page_commit() may not represent a valid value. 

But commit_page is a word length, and I will argue that any compiler that
tears "long" words is broken. ;-)

> 
> In this case, READ_ONCE() is only needed for the commit_page.

But we can at least keep the READ_ONCE() on the commit_page just because it
is used in the next instruction.

-- Steve