linux-kernel - Re: [PATCH 2/3] workqueue: not allow recursion run

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Message-ID: <c62985530901220306p78ea541cs28912a844297b304@mail.gmail.com>
Date:	Thu, 22 Jan 2009 12:06:39 +0100
From:	Frédéric Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	Oleg Nesterov <oleg@...sign.ru>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Eric Dumazet <dada1@...mosbay.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] workqueue: not allow recursion run_workqueue

2009/1/22 Ingo Molnar <mingo@...e.hu>:
>
> * Frederic Weisbecker <fweisbec@...il.com> 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@...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).
>
> WARN_ONCE() is the best method usually - we want a one-time and expressive
> warning, not just a stack dump. (i.e. not WARN_ON and not WARN_ON_ONCE)
>
> Plus some thinking needs to be put into exiting from that function in a way
> that the system will still be usable enough to report the bug.
>
>        Ingo
>

Ok.
Oh but I haven't seen that Oleg said he prefered bug_on, because the
system will deadlock instead....hmm...

Or perhaps keeping the things like the old way, but with a WARN_ONCE:

if (cwq->thread == current) {
               /*
                * Don't ever think to flush workqueue from a work
                */
               WARN_ONCE(1);

               run_workqueue(cwq);
               active = 1;
}

And then, the workqueue will flush...so it will behave correctly but
will warn on this bad developer idea of flushing from a work.

Actually I don't understand when Lai says that it will actually not flush.
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