From: Tim Bird Date: Wed, 3 Jun 2009 00:06:54 +0000 (-0700) Subject: ring-buffer: fix bug in ring_buffer_discard_commit X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=a2023556409cf7fec5d67a26f7fcfa57c5a4086d;p=linux-beck.git ring-buffer: fix bug in ring_buffer_discard_commit There's a bug in ring_buffer_discard_commit. The wrong pointer is being compared in order to check if the event can be freed from the buffer rather than discarded (i.e. marked as PAD). I noticed this when I was working on duration filtering. The bug is not deadly - it just results in lots of wasted space in the buffer. All filtered events are left in the buffer and marked as discarded, rather than being removed from the buffer to make space for other events. Unfortunately, when I fixed this bug, I got errors doing a filtered function trace. Multiple TIME_EXTEND events pile up in the buffer, and trigger the following loop overage warning in rb_iter_peek(): again: ... if (RB_WARN_ON(cpu_buffer, ++nr_loops > 10)) return NULL; I'm not sure what the best way is to fix this. I don't know if I should extend the loop threshhold, or if I should make the test more complex (ignore TIME_EXTEND events), or just get rid of this loop check completely. Note that if I implement a workaround for this, then I see another problem from rb_advance_iter(). I haven't tracked that one down yet. In general, it seems like the case of removing filtered events has not been working properly, and so some assumptions about buffer invariant conditions need to be revisited. Here's the patch for the simple fix: Compare correct pointer for checking if an event can be freed rather than left as discarded in the buffer. Signed-off-by: Tim Bird LKML-Reference: <4A25BE9E.5090909@am.sony.com> Signed-off-by: Steven Rostedt --- diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 16b24d49604c..945302368691 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -1708,7 +1708,7 @@ void ring_buffer_discard_commit(struct ring_buffer *buffer, bpage = cpu_buffer->tail_page; - if (bpage == (void *)addr && rb_page_write(bpage) == old_index) { + if (bpage->page == (void *)addr && rb_page_write(bpage) == old_index) { /* * This is on the tail page. It is possible that * a write could come in and move the tail page