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SubjectRe: [PATCH 2/3] workqueue: not allow recursion run_workqueue
On Thu, Jan 22, 2009 at 10:30:46AM +0100, Frederic Weisbecker wrote:
> On Thu, Jan 22, 2009 at 05:14:24PM +0800, Lai Jiangshan wrote:
> > 1) lockdep will complain when recursion run_workqueue
> > 2) works is not run orderly when recursion run_workqueue
> >
> > 3) BUG!
> > We use recursion run_workqueue to hidden deadlock when
> > keventd trying to flush its own queue.
> >
> > It's bug. When flush_workqueue()(nested in a work callback)returns,
> > the workqueue is not really flushed, the sequence statement of
> > this work callback will do some thing bad.
> >
> > So we should not allow workqueue trying to flush its own queue.
> >
> > Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> > ---
> > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > index 2f44583..1129cde 100644
> > --- a/kernel/workqueue.c
> > +++ b/kernel/workqueue.c
> > @@ -48,8 +48,6 @@ struct cpu_workqueue_struct {
> >
> > struct workqueue_struct *wq;
> > struct task_struct *thread;
> > -
> > - int run_depth; /* Detect run_workqueue() recursion depth */
> > } ____cacheline_aligned;
> >
> > /*
> > @@ -262,13 +260,6 @@ EXPORT_SYMBOL_GPL(queue_delayed_work_on);
> > static void run_workqueue(struct cpu_workqueue_struct *cwq)
> > {
> > spin_lock_irq(&cwq->lock);
> > - cwq->run_depth++;
> > - if (cwq->run_depth > 3) {
> > - /* morton gets to eat his hat */
> > - printk("%s: recursion depth exceeded: %d\n",
> > - __func__, cwq->run_depth);
> > - dump_stack();
> > - }
> > while (!list_empty(&cwq->worklist)) {
> > struct work_struct *work = list_entry(cwq->worklist.next,
> > struct work_struct, entry);
> > @@ -311,7 +302,6 @@ static void run_workqueue(struct cpu_workqueue_struct *cwq)
> > spin_lock_irq(&cwq->lock);
> > cwq->current_work = NULL;
> > }
> > - cwq->run_depth--;
> > spin_unlock_irq(&cwq->lock);
> > }
> >
> > @@ -368,29 +358,20 @@ static void insert_wq_barrier(struct cpu_workqueue_struct *cwq,
> >
> > static int flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)
> > {
> > - int active;
> > + int active = 0;
> > + struct wq_barrier barr;
> >
> > - if (cwq->thread == current) {
> > - /*
> > - * Probably keventd trying to flush its own queue. So simply run
> > - * it by hand rather than deadlocking.
> > - */
> > - run_workqueue(cwq);
> > - active = 1;
> > - } else {
> > - struct wq_barrier barr;
> > + BUG_ON(cwq->thread == current);
>
> Hi Lai,
>
> BUG_ON seems perhaps a bit too much for such case. The system
> will run in an endless loop because of a mistake that will not have
> necessarily a fatal end.
> WARN_ON should be enough (plus the warn that lockdep will raise
> too in this case).
>
> Thanks.
> Frederic.


And perhaps add a comment for the developers who will encounter such a warn,
and then fall down in this call site while searching which warned.
To easily find the reason of the WARN. cwq->thread == current is perhaps not
verbose enough to help the developer finding the source of the problem.

They could solve the issue and say "Doh!" more quickly if they find
in a one shot sight: /* Never flush a workqueue from a work */
:-)


>
> > - active = 0;
> > - spin_lock_irq(&cwq->lock);
> > - if (!list_empty(&cwq->worklist) || cwq->current_work != NULL) {
> > - insert_wq_barrier(cwq, &barr, &cwq->worklist);
> > - active = 1;
> > - }
> > - spin_unlock_irq(&cwq->lock);
> > -
> > - if (active)
> > - wait_for_completion(&barr.done);
> > + spin_lock_irq(&cwq->lock);
> > + if (!list_empty(&cwq->worklist) || cwq->current_work != NULL) {
> > + insert_wq_barrier(cwq, &barr, &cwq->worklist);
> > + active = 1;
> > }
> > + spin_unlock_irq(&cwq->lock);
> > +
> > + if (active)
> > + wait_for_completion(&barr.done);
> >
> > return active;
> > }
> >



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