[PATCH][GIT PULL] tracing/wakeup: move access to wakeup_cpu into spinlock

Steven Rostedt
2009-03-26T14:41:01Z

Ingo,

I believe this is the fix for the oops that Maneesh saw. What he explains
he did sounds like it would trigger the race. Reading the trace output
resets wakeup_task to NULL and wakeup_cpu to -1. And he hit the bug by
just reading the trace in a while loop.

Please pull the latest tip/tracing/ftrace-1 tree, which can be found at:

git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-2.6-trace.git
tip/tracing/ftrace-1

Steven Rostedt (1):
tracing/wakeup: move access to wakeup_cpu into spinlock

----
kernel/trace/trace_sched_wakeup.c | 6 +++---
1 files changed, 3 insertions(+), 3 deletions(-)
---------------------------
commit 8ac070ca952d14c57e3d772e203fd117161cc596
Author: Steven Rostedt <srostedt@redhat.com>
Date: Thu Mar 26 10:25:24 2009 -0400

tracing/wakeup: move access to wakeup_cpu into spinlock

Impact: fix for race condition

The code had the following outside the lock:

if (next != wakeup_task)
return;

pc = preempt_count();

/* The task we are waiting for is waking up */
data = wakeup_trace->data[wakeup_cpu];

On initialization, wakeup_task is NULL and wakeup_cpu -1. This code
is not under a lock. If wakeup_task is set on another CPU as that
task is waking up, we can see the wakeup_task before wakeup_cpu is
set. If we read wakeup_cpu while it is still -1 then we will have
a bad data pointer.

This patch moves the reading of wakeup_cpu within the protection of
the spinlock used to protect the writing of wakeup_cpu and wakeup_task.

Reported-by: Maneesh Soni <maneesh@in.ibm.com>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>

diff --git a/kernel/trace/trace_sched_wakeup.c b/kernel/trace/trace_sched_wakeup.c
index 3c5ad6b..9e4ce4c 100644
--- a/kernel/trace/trace_sched_wakeup.c
+++ b/kernel/trace/trace_sched_wakeup.c
@@ -138,9 +138,6 @@ probe_wakeup_sched_switch(struct rq *rq, struct task_struct *prev,

pc = preempt_count();

- /* The task we are waiting for is waking up */
- data = wakeup_trace->data[wakeup_cpu];
-
/* disable local data, not wakeup_cpu data */
cpu = raw_smp_processor_id();
disabled = atomic_inc_return(&wakeup_trace->data[cpu]->disabled);
@@ -154,6 +151,9 @@ probe_wakeup_sched_switch(struct rq *rq, struct task_struct *prev,
if (unlikely(!tracer_enabled || next != wakeup_task))
goto out_unlock;

+ /* The task we are waiting for is waking up */
+ data = wakeup_trace->data[wakeup_cpu];
+
trace_function(wakeup_trace, CALLER_ADDR1, CALLER_ADDR2, flags, pc);
tracing_sched_switch_trace(wakeup_trace, prev, next, flags, pc);