On Fri, 8 Feb 2019 15:49:14 +0200
Tzvetomir Stoyanov <tstoyanov@xxxxxxxxxx> wrote:
> Fixed the logic in tracecmd_read_page_record() when
> searching for the next event in the event from the
> ring buffer. In the current logic, in the case when
> last_record is not NULL (we are searching for the second one)
> the check "if (ptr != last_record->data)" always fails.
>
Nice catch.
Let's change the change log and subject:
Subject trace-cmd: Fix tracecmd_read_page_record() to read more than one event
The kbuffer_next_event() will return the next event on the sub buffer.
If we pass in the last_record to tracecmd_read_page_record(), it
initializes the sub buffer, and by calling kbuffer_next_event()
(instead of kbuffer_read_event()), the second event on the sub buffer
is returned. This causes the match of the last_record not to match if
the last_record happens to be the first event on the sub buffer.
> Signed-off-by: Tzvetomir Stoyanov <tstoyanov@xxxxxxxxxx>
> ---
> lib/trace-cmd/trace-input.c | 12 ++++++++----
> 1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/lib/trace-cmd/trace-input.c b/lib/trace-cmd/trace-input.c
> index 33ad7d1..5d64ca8 100644
> --- a/lib/trace-cmd/trace-input.c
> +++ b/lib/trace-cmd/trace-input.c
> @@ -1688,18 +1688,22 @@ tracecmd_read_page_record(struct tep_handle *pevent, void *page, int size,
> goto out_free;
> }
>
> - do {
> + ptr = kbuffer_read_event(kbuf, &ts);
> + while (ptr < last_record->data) {
> ptr = kbuffer_next_event(kbuf, NULL);
> if (!ptr)
> break;
> - } while (ptr < last_record->data);
> + if (ptr == last_record->data)
> + break;
> + }
> if (ptr != last_record->data) {
> warning("tracecmd_read_page_record: could not find last_record");
> goto out_free;
> }
> - }
> + ptr = kbuffer_next_event(kbuf, &ts);
> + } else
> + ptr = kbuffer_read_event(kbuf, &ts);
>
> - ptr = kbuffer_read_event(kbuf, &ts);
Also, instead of the above "} else" clause, we can just move the
ptr = kbuffer_read_event(kbuf, &ts);
before the if condition testing last_record.
-- Steve
> if (!ptr)
> goto out_free;
>
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