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Subject[PATCH 1/2] workqueue: Catch more locking problems with flush_work()
Date
If a workqueue is flushed but the work item is not scheduled to
run, lockdep checking will be circumvented. For example:

static DEFINE_MUTEX(mutex);

static void my_work(struct work_struct *w)
{
mutex_lock(&mutex);
mutex_unlock(&mutex);
}
static DECLARE_WORK(work, my_work);

static int __init start_test_module(void)
{
schedule_work(&work);
return 0;
}
module_init(start_test_module);
static void __exit stop_test_module(void)
{
mutex_lock(&mutex);
flush_work(&work);
mutex_unlock(&mutex);
}
module_exit(stop_test_module);
would only print a warning if the work item was actively running
when flush_work() was called. Otherwise flush_work() returns
early. In this trivial example nothing could go wrong, but if the
work item is schedule via an interrupt we could potentially have a
scenario where the work item is running just at the time flush_work()
is called. This could become a classic AB-BA locking problem.

Add a lockdep hint in flush_work() in the "work not running" case
so that we always catch this potential deadlock scenario.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
---
kernel/workqueue.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 66ec08d..eb800df 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2513,8 +2513,11 @@ bool flush_work(struct work_struct *work)
wait_for_completion(&barr.done);
destroy_work_on_stack(&barr.work);
return true;
- } else
+ } else {
+ lock_map_acquire(&work->lockdep_map);
+ lock_map_release(&work->lockdep_map);
return false;
+ }
}
EXPORT_SYMBOL_GPL(flush_work);

--
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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