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SubjectRe: [BUG] 2.6.28-git LOCKDEP: Possible recursive rq->lock
* Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com> [2009-01-07 22:01:00]:

> * Peter Zijlstra <a.p.zijlstra@chello.nl> [2009-01-07 15:28:57]:
>
> > On Wed, 2009-01-07 at 19:50 +0530, Vaidyanathan Srinivasan wrote:
> > > * Peter Zijlstra <a.p.zijlstra@chello.nl> [2009-01-07 14:12:43]:
> > >
> > > > On Wed, 2009-01-07 at 17:59 +0530, Vaidyanathan Srinivasan wrote:
> > > >
> > > > > =============================================
> > > > > [ INFO: possible recursive locking detected ]
> > > > > 2.6.28-autotest-tip-sv #1
> > > > > ---------------------------------------------
> > > > > klogd/5062 is trying to acquire lock:
> > > > > (&rq->lock){++..}, at: [<ffffffff8022aca2>] task_rq_lock+0x45/0x7e
> > > > >
> > > > > but task is already holding lock:
> > > > > (&rq->lock){++..}, at: [<ffffffff805f7354>] schedule+0x158/0xa31
> > > > >
> > > > > other info that might help us debug this:
> > > > > 1 lock held by klogd/5062:
> > > > > #0: (&rq->lock){++..}, at: [<ffffffff805f7354>] schedule+0x158/0xa31
> > > > >
> > > > > stack backtrace:
> > > > > Pid: 5062, comm: klogd Not tainted 2.6.28-autotest-tip-sv #1
> > > > > Call Trace:
> > > > > [<ffffffff80259ef1>] __lock_acquire+0xeb9/0x16a4
> > > > > [<ffffffff8025a6c0>] ? __lock_acquire+0x1688/0x16a4
> > > > > [<ffffffff8025a761>] lock_acquire+0x85/0xa9
> > > > > [<ffffffff8022aca2>] ? task_rq_lock+0x45/0x7e
> > > > > [<ffffffff805fa4d4>] _spin_lock+0x31/0x66
> > > > > [<ffffffff8022aca2>] ? task_rq_lock+0x45/0x7e
> > > > > [<ffffffff8022aca2>] task_rq_lock+0x45/0x7e
> > > > > [<ffffffff80233363>] try_to_wake_up+0x88/0x27a
> > > > > [<ffffffff80233581>] wake_up_process+0x10/0x12
> > > > > [<ffffffff805f775c>] schedule+0x560/0xa31
> > > >
> > > > I'd be most curious to know where in schedule we are.
> > >
> > > ok, we are in sched.c:3777
> > >
> > > double_unlock_balance(this_rq, busiest);
> > > if (active_balance)
> > > >>>>>>>>>>> wake_up_process(busiest->migration_thread);
> > >
> > > } else
> > >
> > > In active balance in newidle. This implies sched_mc was 2 at that time.
> > > let me trace this and debug further.
> >
> > How about something like this? Strictly speaking we'll not deadlock,
> > because ttwu will not be able to place the migration task on our rq, but
> > since the code can deal with both rqs getting unlocked, this seems the
> > easiest way out.
>
> Hi Peter,
>
> I agree. Unlocking this_rq is an easy way out. Thanks for the
> suggestion. I have moved the unlock and lock withing the if
> condition.
>
> --Vaidy
>
> sched: bug fix -- do not call ttwu while holding rq->lock
>
> When sched_mc=2 wake_up_process() is called on busiest_rq
> while holding this_rq lock in load_balance_newidle()
> Though this will not deadlock, this is a lockdep warning
> and the situation is easily solved by releasing the this_rq
> lock at this point in code
>
> Signed-off-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 71a054f..703a669 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -3773,8 +3773,12 @@ redo:
> }
>
> double_unlock_balance(this_rq, busiest);
> - if (active_balance)
> + if (active_balance) {
> + /* Should not call ttwu while holding a rq->lock */
> + spin_unlock(&this_rq->lock);
> wake_up_process(busiest->migration_thread);
> + spin_lock(&this_rq->lock);
> + }
>
> } else
> sd->nr_balance_failed = 0;


Hi Peter and Ingo,

The above fix seem to have fixed the lockdep warning. Please include
in sched-tip for further testing and later push to mainline.

Thanks,
Vaidy



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