[PATCHv2] workqueue: Catch more locking problems with flush_work()

From: Stephen Boyd
Date: Fri Apr 20 2012 - 20:28:49 EST

If a workqueue is flushed with flush_work() lockdep checking can
be circumvented. For example:

static DEFINE_MUTEX(mutex);

static void my_work(struct work_struct *w)

static DECLARE_WORK(work, my_work);

static int __init start_test_module(void)
return 0;

static void __exit stop_test_module(void)

would not always print a warning when flush_work() was called.
In this trivial example nothing could go wrong since we are
guaranteed module_init() and module_exit() don't run concurrently,
but if the work item is schedule asynchronously we could have a
scenario where the work item is running just at the time flush_work()
is called resulting in a classic ABBA locking problem.

Add a lockdep hint by acquiring and releasing the work item
lockdep_map in flush_work() so that we always catch this
potential deadlock scenario.

Signed-off-by: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
kernel/workqueue.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 5abf42f..038cf64 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2506,6 +2506,9 @@ bool flush_work(struct work_struct *work)
struct wq_barrier barr;

+ lock_map_acquire(&work->lockdep_map);
+ lock_map_release(&work->lockdep_map);
if (start_flush_work(work, &barr, true)) {
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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