Re: races when reserving an event in the unified trace buffer

[Steven Rostedt]

Hi Jiaying,

Sorry for taking so long to reply.

On Fri, 2008-12-12 at 16:26 -0800, Jiaying Zhang wrote:
> Hi Steve,
> 
> I am doing some load testing with our kernel tracing prototype
> that uses the unified trace buffer for managing its data. I sometimes
> saw kernel stack dump caused by the following checking in
> function  __rb_reserve_next:
>         if (unlikely(next_page == cpu_buffer->commit_page)) {
>                  WARN_ON_ONCE(1);
>                  goto out_unlock;
>        }
> The comments above the code say the problem is caused by
> "an interrupt storm that made it all the way around the buffer".
> But I think there is race here that a single interrupt can cause
> the check to fail. Suppose this is what happens:
> An event is traced and calls __rb_reserve_next. Right after it
> gets the current tail_page (line tail_page = cpu_buffer->tail_page;),
> an interrupt happens that is also traced. The interrupt also takes
> the same tail_page. The interrupt event moves the tail_page
> forward if the tail_page is full. Note that the interrupt event gets
> the old 'write' value because the first event has not updated that yet.

Good catch.

> So the interrupt event may also update the commit_page if it is
> the same as the tail_page. As a result, the above check would
> fail after the interrupt finishes and the first event resumes its execution.
> 
> I have seen the problem happens frequently under heavy loads
> on a multi-core machine. Interestingly, I also saw the above
> warning that might actually be caused by an interrupt storm.
> I was using 64k buffer size and am not sure whether it is possible
> for so many interrupts to happen in a short time window.
> 
> I think we can use the time_stamp to distinguish the two cases.
> Also, in either case, it seems bad to leave the tail_page->write with
> an invalid value because it can cause problem when a reader
> reads the page. Here is my proposed fix for the problem:
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 7f69cfe..1500f78 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -982,8 +982,11 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
>                  * it all the way around the buffer, bail, and warn
>                  * about it.
>                  */
> -               if (unlikely(next_page == cpu_buffer->commit_page)) {
> +               if (unlikely(next_page == cpu_buffer->commit_page) &&
> +                               tail_page->time_stamp > next_page->time_stamp) {
>                         WARN_ON_ONCE(1);
> +                       if (tail <= BUF_PAGE_SIZE)
> +                               local_set(&tail_page->write, tail);

Actually what we probably should do instead, is simply record the commit
page first:

+	commit_page = cpu_buffer->commit_page;
+	barrier();
	tail_page = cpu_buffer->tail_page;
	write = local_add_return(length, &tail_page->write);


And then we could test next_page == commit_page instead.

-- Steve


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