Re: [PATCH] ring-buffer: use READ_ONCE() to read cpu_buffer->commit_page in concurrent environment

Steven Rostedt
2024-03-01T15:47:37Z

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