linux-kernel - [tip:tracing/core] ring-buffer: only warn on wrap if buffer is bigger than two pages

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Date:	Tue, 21 Apr 2009 14:03:29 GMT
From:	tip-bot for Steven Rostedt <srostedt@...hat.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...hat.com,
	rostedt@...dmis.org, srostedt@...hat.com, tglx@...utronix.de,
	mingo@...e.hu
Subject: [tip:tracing/core] ring-buffer: only warn on wrap if buffer is bigger than two pages

Commit-ID:  3554228d4289098a8fe5cfd87512ec32a19bbe5a
Gitweb:     http://git.kernel.org/tip/3554228d4289098a8fe5cfd87512ec32a19bbe5a
Author:     Steven Rostedt <srostedt@...hat.com>
AuthorDate: Tue, 21 Apr 2009 09:41:26 -0400
Committer:  Ingo Molnar <mingo@...e.hu>
CommitDate: Tue, 21 Apr 2009 16:00:45 +0200

ring-buffer: only warn on wrap if buffer is bigger than two pages

On boot up, to save memory, ftrace allocates the minimum buffer
which is two pages. Ftrace also goes through a series of tests
(when configured) on boot up. These tests can fill up a page within
a single interrupt.

The ring buffer also has a WARN_ON when it detects that the buffer was
completely filled within a single commit (other commits are allowed to
be nested).

Combine the small buffer on start up, with the tests that can fill more
than a single page within an interrupt, this can trigger the WARN_ON.

This patch makes the WARN_ON only happen when the ring buffer consists
of more than two pages.

[ Impact: prevent false WARN_ON in ftrace startup tests ]

Reported-by: Ingo Molnar <mingo@...e.hu>
LKML-Reference: <20090421094616.GA14561@...e.hu>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Ingo Molnar <mingo@...e.hu>


---
 kernel/trace/ring_buffer.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 7bcfd3e..61dbdf2 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1241,7 +1241,8 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
 		 * about it.
 		 */
 		if (unlikely(next_page == commit_page)) {
-			WARN_ON_ONCE(1);
+			/* This can easily happen on small ring buffers */
+			WARN_ON_ONCE(buffer->pages > 2);
 			goto out_reset;
 		}
 
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