[PATCH] Fix race in ring_buffer_consume()

[Indan Zupancic]

Hello,

While doing a grep -r as root somewhere in /proc, grep founds its way
into /mnt/debug/tracing and caused the below snippet:

------------[ cut here ]------------
kernel BUG at /home/indan/src/linux-2.6/kernel/trace/ring_buffer.c:1676!
invalid opcode: 0000 [#1] PREEMPT
last sysfs file: /sys/devices/virtual/backlight/thinkpad_screen/brightness
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in: i915 drm pl2303 usbserial usb_storage uhci_hcd ehci_hcd
usbcore

Pid: 10660, comm: grep Not tainted (2.6.28phc #12) 2371GHG
EIP: 0060:[<c0241464>] EFLAGS: 00010246 CPU: 0
EIP is at rb_advance_reader+0xe/0xd7
EAX: 00000000 EBX: f703c000 ECX: f717ea80 EDX: 00000003
ESI: f70081c0 EDI: f7008240 EBP: 00000fff ESP: f6ac2f3c
 DS: 007b ES: 007b FS: 0000 GS: 0033 SS: 0068
Process grep (pid: 10660, ti=f6ac2000 task=f689b900 task.ti=f6ac2000)
Stack:
 f703c000 f70081c0 00000000 c024171e f2de2000 f2de2000 f2de3038 c0242fc8
 00000a6a c0245105 089adff4 f2dca400 f2de2010 00005000 f2dca400 c0244f8f
 089adff4 c0263633 f6ac2fa0 f2dca400 fffffff7 00001000 f6ac2000 c02636f9
Call Trace:
 [<c024171e>] ring_buffer_consume+0x2b/0x31
 [<c0242fc8>] trace_consume+0x1d/0x23
 [<c0245105>] tracing_read_pipe+0x176/0x1dc
 [<c0244f8f>] tracing_read_pipe+0x0/0x1dc
 [<c0263633>] vfs_read+0x73/0xa1
 [<c02636f9>] sys_read+0x3c/0x63
 [<c0202ef5>] sysenter_do_call+0x12/0x2a
Code: 00 25 00 f0 ff ff e8 f9 fb ff ff 85 c0 74 05 e8 8c 61 1c 00
89 f0 5e 5f 5b 5e 5f 5d c3 57 89 c7 56 53 e8 c2 fe ff ff 85 c0 75
04 <0f> 0b eb fe 8b 47 1c 8b 70 1c 03 70 10 8a 06 24 03 3c 03 75 03
EIP: [<c0241464>] rb_advance_reader+0xe/0xd7 SS:ESP 0068:f6ac2f3c
---[ end trace 77bf081d52bacbc7 ]---
note: grep[10660] exited with preempt_count 1

I think preemptirqsoff tracing is enabled, can't check because doing
cat on most files in debug/tracing hangs. Kernel was booted with
initcall_debug. Kernel is 2.6.28.

$ zcat /proc/config.gz |grep TRACER=
CONFIG_NOP_TRACER=y
CONFIG_HAVE_FUNCTION_TRACER=y
CONFIG_IRQSOFF_TRACER=y
CONFIG_PREEMPT_TRACER=y
CONFIG_CONTEXT_SWITCH_TRACER=y
CONFIG_BOOT_TRACER=y

ring_buffer.c:1676:

static void rb_advance_reader(struct ring_buffer_per_cpu *cpu_buffer)
{
	struct ring_buffer_event *event;
	struct buffer_page *reader;
	unsigned length;

	reader = rb_get_reader_page(cpu_buffer);

	/* This function should not be called when buffer is empty */
	BUG_ON(!reader);

Called by ring_buffer_consume():

ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts)
{
	struct ring_buffer_per_cpu *cpu_buffer;
	struct ring_buffer_event *event;

	if (!cpu_isset(cpu, buffer->cpumask))
		return NULL;

	event = ring_buffer_peek(buffer, cpu, ts);
	if (!event)
		return NULL;

	cpu_buffer = buffer->buffers[cpu];
	rb_advance_reader(cpu_buffer);

ring_buffer_peek() is complicated, but boils down to:

ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts)
{
[...]
	cpu_buffer = buffer->buffers[cpu];
[...]
	reader = rb_get_reader_page(cpu_buffer);
	if (!reader)
		return NULL;

	event = rb_reader_event(cpu_buffer);
[...]
	return event or return NULL;
}

So between the rb_get_reader_page() call in ring_buffer_peek() and
the one in rb_advance_reader() something happened causing the event
to disappear. Also note: grep[10660] exited with preempt_count 1.
There's no locking between the ring_buffer_peek and rb_advance_reader
calls in ring_buffer_consume, so a tentative guess is that there should
be some.

Then again, the double tracing_read_pipe() in the backtrace seems weird,
so maybe something else is going on.

Easiest fix seems to merge ring_buffer_peek and ring_buffer_consume into
ring_buffer_get, which is the same as the current peek, but with an extra
parameter telling it to consume the buffer or not.

The same might be needed for ring_buffer_iter_peek() and ring_buffer_read().

Patch doing this:

commit 648cf957210619595856b78f12bd11752ae22aa5
Author: Indan Zupancic <indan@xxxxxx>
Date:   Mon Dec 29 14:27:18 2008 +1100

    Fix race in ring_buffer_consume(): Replace ring_buffer_consume and
ring_buffer_peek with ring_buffer_get_event

    Signed-off-by: Indan Zupancic <indan@xxxxxx>

 include/linux/ring_buffer.h   |    4 +---
 kernel/trace/ring_buffer.c    |   39 ++++++++-------------------------------
 kernel/trace/trace.c          |   15 ++++++++-------
 kernel/trace/trace_selftest.c |    2 +-
 4 files changed, 18 insertions(+), 42 deletions(-)

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index e097c2e..d9320bd 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -83,9 +83,7 @@ int ring_buffer_write(struct ring_buffer *buffer,
 		      unsigned long length, void *data);

 struct ring_buffer_event *
-ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts);
-struct ring_buffer_event *
-ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts);
+ring_buffer_get_event(struct ring_buffer *buffer, int cpu, u64 *ts, int
consume);

 struct ring_buffer_iter *
 ring_buffer_read_start(struct ring_buffer *buffer, int cpu);
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 668bbb5..d4e5dbc 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1727,16 +1727,18 @@ static void rb_advance_iter(struct ring_buffer_iter
*iter)
 }

 /**
- * ring_buffer_peek - peek at the next event to be read
+ * ring_buffer_get_event - get the next event to be read
  * @buffer: The ring buffer to read
  * @cpu: The cpu to peak at
  * @ts: The timestamp counter of this event.
+ * @consume: Whether the event should be consumed.
+ * If true, sequential reads will keep returning a different event,
+ * and eventually empty the ring buffer if the producer is slower.
  *
- * This will return the event that will be read next, but does
- * not consume the data.
+ * This will return the event that will be read next.
  */
 struct ring_buffer_event *
-ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts)
+ring_buffer_get_event(struct ring_buffer *buffer, int cpu, u64 *ts, int consume)
 {
 	struct ring_buffer_per_cpu *cpu_buffer;
 	struct ring_buffer_event *event;
@@ -1789,6 +1791,8 @@ ring_buffer_peek(struct ring_buffer *buffer, int cpu,
u64 *ts)
 			*ts = cpu_buffer->read_stamp + event->time_delta;
 			ring_buffer_normalize_time_stamp(cpu_buffer->cpu, ts);
 		}
+		if (consume)
+			rb_advance_reader(cpu_buffer);
 		return event;

 	default:
@@ -1869,33 +1873,6 @@ ring_buffer_iter_peek(struct ring_buffer_iter *iter,
u64 *ts)
 }

 /**
- * ring_buffer_consume - return an event and consume it
- * @buffer: The ring buffer to get the next event from
- *
- * Returns the next event in the ring buffer, and that event is consumed.
- * Meaning, that sequential reads will keep returning a different event,
- * and eventually empty the ring buffer if the producer is slower.
- */
-struct ring_buffer_event *
-ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts)
-{
-	struct ring_buffer_per_cpu *cpu_buffer;
-	struct ring_buffer_event *event;
-
-	if (!cpu_isset(cpu, buffer->cpumask))
-		return NULL;
-
-	event = ring_buffer_peek(buffer, cpu, ts);
-	if (!event)
-		return NULL;
-
-	cpu_buffer = buffer->buffers[cpu];
-	rb_advance_reader(cpu_buffer);
-
-	return event;
-}
-
-/**
  * ring_buffer_read_start - start a non consuming read of the buffer
  * @buffer: The ring buffer to read from
  * @cpu: The cpu buffer to iterate over
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index d86e325..f1329ed 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -938,7 +938,7 @@ peek_next_entry(struct trace_iterator *iter, int cpu, u64
*ts)
 	if (buf_iter)
 		event = ring_buffer_iter_peek(buf_iter, ts);
 	else
-		event = ring_buffer_peek(iter->tr->buffer, cpu, ts);
+		event = ring_buffer_get_event(iter->tr->buffer, cpu, ts, 0);

 	ftrace_enable_cpu();

@@ -1002,7 +1002,7 @@ static void trace_consume(struct trace_iterator *iter)
 {
 	/* Don't allow ftrace to trace into the ring buffers */
 	ftrace_disable_cpu();
-	ring_buffer_consume(iter->tr->buffer, iter->cpu, &iter->ts);
+	ring_buffer_get_event(iter->tr->buffer, iter->cpu, &iter->ts, 1);
 	ftrace_enable_cpu();
 }

@@ -1310,8 +1310,9 @@ void trace_seq_print_cont(struct trace_seq *s, struct
trace_iterator *iter)
 	struct trace_entry *ent;
 	struct trace_field_cont *cont;
 	bool ok = true;
+	int cpu = iter->cpu;

-	ent = peek_next_entry(iter, iter->cpu, NULL);
+	ent = peek_next_entry(iter, cpu, NULL);
 	if (!ent || ent->type != TRACE_CONT) {
 		trace_seq_putc(s, '\n');
 		return;
@@ -1324,14 +1325,14 @@ void trace_seq_print_cont(struct trace_seq *s, struct
trace_iterator *iter)

 		ftrace_disable_cpu();

-		if (iter->buffer_iter[iter->cpu])
-			ring_buffer_read(iter->buffer_iter[iter->cpu], NULL);
+		if (iter->buffer_iter[cpu])
+			ring_buffer_read(iter->buffer_iter[cpu], NULL);
 		else
-			ring_buffer_consume(iter->tr->buffer, iter->cpu, NULL);
+			ring_buffer_get_event(iter->tr->buffer, cpu, NULL, 1);

 		ftrace_enable_cpu();

-		ent = peek_next_entry(iter, iter->cpu, NULL);
+		ent = peek_next_entry(iter, cpu, NULL);
 	} while (ent && ent->type == TRACE_CONT);

 	if (!ok)
diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c
index 90bc752..3894989 100644
--- a/kernel/trace/trace_selftest.c
+++ b/kernel/trace/trace_selftest.c
@@ -23,7 +23,7 @@ static int trace_test_buffer_cpu(struct trace_array *tr, int
cpu)
 	struct ring_buffer_event *event;
 	struct trace_entry *entry;

-	while ((event = ring_buffer_consume(tr->buffer, cpu, NULL))) {
+	while ((event = ring_buffer_get_event(tr->buffer, cpu, NULL, 1))) {
 		entry = ring_buffer_event_data(event);

 		if (!trace_valid_entry(entry)) {


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