On Sun, Jun 01, 2014 at 10:08:21AM -0400, Sasha Levin wrote: > On 05/12/2014 12:28 PM, Jan Kara wrote: > > On Wed 07-05-14 22:03:08, Sasha Levin wrote: > >> > Hi all, > >> > > >> > While fuzzing with trinity inside a KVM tools guest running the latest -next > >> > kernel I've stumbled on the following spew: > > Thanks for report. So the problem seems to be maginally valid but I'm not > > 100% sure whom to blame :). So printk() code calls up() which calls > > try_to_wake_up() under console_sem.lock spinlock. That function can take > > rq->lock which is all expected. > > > > The next part of the chain is that during CPU initialization we call > > __sched_fork() with rq->lock which calls into hrtimer_init() which can > > allocate memory which creates a dependency rq->lock => zone.lock.rlock. > > > > And memory management code calls printk() which zone.lock.rlock held which > > closes the loop. Now I suspect the second link in the chain can happen only > > while CPU is booting and might even happen only if some debug options are > > enabled. But I don't really know scheduler code well enough. Steven? > > I've cc'ed Peter and Ingo who may be able to answer that, as it still happens > on -next. Ah, cute. So the second paragraph seems to miss the detail that this is the __sched_fork() call from init_idle(), all other callers don't actually hold the rq->lock. Now init_idle() is called from: sched_init() fork_idle() idle_thread_get() Now fork_idle() is called from: smp_init() -> idle_threads_init() -> idle_init() and idle_thread_get is called from: _cpu_up() So while it looks we're calling __sched_fork() twice for every !boot idle thread (urgh) we do appear to call it before anything is running on that cpu, so I don't see any particular problem with removing the call from under that lock. Something like so should do I suppose. --- kernel/sched/core.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 240aa83e73f5..99609c33482b 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4505,9 +4505,10 @@ void init_idle(struct task_struct *idle, int cpu) struct rq *rq = cpu_rq(cpu); unsigned long flags; + __sched_fork(0, idle); + raw_spin_lock_irqsave(&rq->lock, flags); - __sched_fork(0, idle); idle->state = TASK_RUNNING; idle->se.exec_start = sched_clock();
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