ring-buffer: pass in lockdep class key for reader_lock

On Sun, 7 Jun 2009, Ingo Molnar wrote:
> Testing tracer sched_switch: <6>Starting ring buffer hammer
> PASSED
> Testing tracer sysprof: PASSED
> Testing tracer function: PASSED
> Testing tracer irqsoff:
> =============================================
> PASSED
> Testing tracer preemptoff: PASSED
> Testing tracer preemptirqsoff: [ INFO: possible recursive locking detected ]
> PASSED
> Testing tracer branch: 2.6.30-rc8-tip-01972-ge5b9078-dirty #5760
> ---------------------------------------------
> rb_consumer/431 is trying to acquire lock:
>  (&cpu_buffer->reader_lock){......}, at: [<c109eef7>] ring_buffer_reset_cpu+0x37/0x70
>
> but task is already holding lock:
>  (&cpu_buffer->reader_lock){......}, at: [<c10a019e>] ring_buffer_consume+0x7e/0xc0
>
> other info that might help us debug this:
> 1 lock held by rb_consumer/431:
>  #0:  (&cpu_buffer->reader_lock){......}, at: [<c10a019e>] ring_buffer_consume+0x7e/0xc0

The ring buffer is a generic structure, and can be used outside of
ftrace. If ftrace traces within the use of the ring buffer, it can produce
false positives with lockdep.

This patch passes in a static lock key into the allocation of the ring
buffer, so that different ring buffers will have their own lock class.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1244477919.13761.9042.camel@twins>

[ store key in ring buffer descriptor ]

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 13 insertions, 1 deletions

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index f1345828c7c5..8670f1575fe1 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -105,7 +105,19 @@ void ring_buffer_discard_commit(struct ring_buffer *buffer,
  * size is in bytes for each per CPU buffer.
  */
 struct ring_buffer *
-ring_buffer_alloc(unsigned long size, unsigned flags);
+__ring_buffer_alloc(unsigned long size, unsigned flags, struct lock_class_key *key);
+
+/*
+ * Because the ring buffer is generic, if other users of the ring buffer get
+ * traced by ftrace, it can produce lockdep warnings. We need to keep each
+ * ring buffer's lock class separate.
+ */
+#define ring_buffer_alloc(size, flags)			\
+({							\
+	static struct lock_class_key __key;		\
+	__ring_buffer_alloc((size), (flags), &__key);	\
+})
+
 void ring_buffer_free(struct ring_buffer *buffer);
 
 int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size);