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SubjectRe: [PATCH v2 1/1] ftrace: do not update max buffer with no users

On Thu, 13 Nov 2008, Ingo Molnar wrote:

>
> * Steven Rostedt <rostedt@goodmis.org> wrote:
>
> >
> > On Thu, 13 Nov 2008, Ingo Molnar wrote:
> > >
> > > the obvious solution is to add this to ring_buffer_resize():
> > >
> > > if (!buffer)
> > > return size;
> >
> > Having a NULL buffer return a successful resize is a bit worrisome to me.
> >
> > And looking at the code I was trying to make sure could never be called
> > if there are no max_tr users:
> >
> > in update_max_tr
> >
> > buf = tr->buffer;
> > tr->buffer = max_tr.buffer;
> > max_tr.buffer = buf;
> >
> > Should all the ring buffer API return success on NULL pointers?
>
> well, update_max_tr() is only used from tracers which also actually
> allocate a max trace buffer: irqs/preemptoff, sched-wakeup.

Which is what we expect. But just the fact that there's nothing protecting
update_max_tr from being used by anything else means we really can not
prove that those are the only three users, without reviewing the code.


>
> So, can you tell me any way how the patch below could be broken?

I just do not see how we can say resizing a NULL pointer to a given size
succeeds without actually resizing anything.

From the comment above the code:

/**
* ring_buffer_resize - resize the ring buffer
* @buffer: the buffer to resize.
* @size: the new size.
*
* The tracer is responsible for making sure that the buffer is
* not being used while changing the size.
* Note: We may be able to change the above requirement by using
* RCU synchronizations.
*
* Minimum size is 2 * BUF_PAGE_SIZE.
*
* Returns -1 on failure.
*/


To me, the correct response to !buffer is a -1, not size. And a -1 would
make the ftrace code fail. At least it would not crash.

-- Steve


>
> Ingo
>
> ------------->
> >From ee51a1de7e3837577412be269e0100038068e691 Mon Sep 17 00:00:00 2001
> From: Ingo Molnar <mingo@elte.hu>
> Date: Thu, 13 Nov 2008 14:58:31 +0100
> Subject: [PATCH] tracing: fix mmiotrace resizing crash
>
> Pekka reported a crash when resizing the mmiotrace tracer (if only
> mmiotrace is enabled).
>
> This happens because in that case we do not allocate the max buffer,
> but we try to use it.
>
> Make ring_buffer_resize() idempotent against NULL buffers.
>
> Reported-by: Pekka Paalanen <pq@iki.fi>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
> ---
> kernel/trace/ring_buffer.c | 6 ++++++
> 1 files changed, 6 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 231db20..036456c 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -538,6 +538,12 @@ int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size)
> LIST_HEAD(pages);
> int i, cpu;
>
> + /*
> + * Always succeed at resizing a non-existent buffer:
> + */
> + if (!buffer)
> + return size;
> +
> size = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
> size *= BUF_PAGE_SIZE;
> buffer_size = buffer->pages * BUF_PAGE_SIZE;
>
>


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