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

linke
2024-02-29T12:37:46Z

Hi Steven, sorry for the late reply.

>
> Now the reason for the above READ_ONCE() is because the variables *are*
> going to be used again. We do *not* want the compiler to play any games
> with that.
>

I don't think it is because the variables are going to be used again.
Compiler optimizations barely do bad things in single thread programs. It
is because cpu_buffer->commit_page may change concurrently and should be
accessed atomically.

/* Make sure commit page didn't change */
curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);

/* If the commit page changed, then there's more data */
if (curr_commit_page != commit_page ||
curr_commit_ts != commit_ts)
return 0;

This code read cpu_buffer->commit_page and time_stamp again to check
whether commit page changed. It shows that cpu_buffer->commit_page and
time_stamp may be changed by other threads.

commit_page = cpu_buffer->commit_page;
commit_ts = commit_page->page->time_stamp;

So the commit_page and time_stamp above is read while other threads may
change it. I think it is a data race if it is not atomic. Thus it is
necessary to use READ_ONCE() here.