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

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Message-ID: <tencent_3F31ADAB6142D8948E5BD02B854ED2A77106@qq.com>
Date: Fri, 1 Mar 2024 13:37:18 +0800
From: linke <lilinke99@...com>
To: rostedt@...dmis.org
Cc: lilinke99@...com,
 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

> 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.

> 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. 

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