Messages in this thread | | | Subject | Re: Warning from swake_up_all in 4.14.15-rt13 non-RT | From | Corey Minyard <> | Date | Tue, 6 Mar 2018 16:51:53 -0600 |
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On 03/06/2018 11:46 AM, Sebastian Andrzej Siewior wrote: > On 2018-03-05 09:08:11 [-0600], Corey Minyard wrote: >> Starting with the change >> >> 8a64547a07980f9d25e962a78c2e10ee82bdb742 fs/dcache: use swait_queue instead >> of >> waitqueue > … >> The following change is the obvious reason: >> >> --- a/kernel/sched/swait.c >> +++ b/kernel/sched/swait.c >> @@ -69,6 +69,7 @@ void swake_up_all(struct swait_queue_head *q) >> struct swait_queue *curr; >> LIST_HEAD(tmp); >> >> + WARN_ON(irqs_disabled()); >> raw_spin_lock_irq(&q->lock); >> list_splice_init(&q->task_list, &tmp); >> while (!list_empty(&tmp)) { >> >> I've done a little bit of analysis here, percpu_ref_kill_and_confirm() >> does spin_lock_irqsave() and then does a percpu_ref_put(). If the >> refcount reaches zero, the release function of the refcount is >> called. In this case, the block code has set this to >> blk_queue_usage_counter_release(), which calls swake_up_all(). >> >> It seems like a bad idea to call percpu_ref_put() with interrupts >> disabled. This problem actually doesn't appear to be RT-related, >> there's just no warning call if the RT tree isn't used. > yeah but vanilla uses wake_up() which does spin_lock_irqsafe() so it is > not an issue there. > > The odd part here is that percpu_ref_kill_and_confirm() does _irqsave() > which suggests that it might be called from any context and then it does > wait_event_lock_irq() which enables interrupts again while it waits. So > it can't be used from any context.
I'm adding the author (Kent) to this email, I should have done that originally.
You are right, it looks like all the percpu_ref_switch.. and percpu_ref_kill... functions are broken here.
I also don't understand the need for a global lock for non-global variables. It looks like this could become a bottleneck in a big SMP system.
I'm going to spend some time with this and try to figure out what is going on. Hopefully Kent or Tejun can offer some insight.
-corey
>> I'm not sure if it's best to just do the put outside the lock, or >> have modified put function that returns a bool to know if a release >> is required, then the release function can be called outside the >> lock. I can do patches and test, but I'm hoping for a little >> guidance here. > swake_up_all() does raw_spin_lock_irq() because it should be called from > non-IRQ context. And it drops the lock (+IRQ enable) between wake-ups in > case we "need_resched()" because we woke a high-priority waiter. There > is the list_splice because we wanted to drop the locks (and have IRQs > open) during the entire wake up process but finish_swait() may happen > during the wake up and so we must hold the lock while the list-item is > removed for the queue head. > I have no idea what is the wisest thing to do here. The obvious fix > would be to use the irqsafe() variant here and not drop the lock between > wake ups. That is essentially what swake_up_all_locked() does which I > need for the completions (and based on some testing most users have one > waiter except during PM and some crypto code). > It is probably no comparison to wake_up_q() (which does multiple wake > ups without a context switch) but then we did this before like that. > > Preferably we would have a proper list_splice() and some magic in the > "early" dequeue part that works. > >> I'm also wondering why we don't have a warning like this in the >> *_spin_lock_irq() macros, perhaps turned on with a debug >> option. That would catch things like this sooner. > Ideally you would add lockdep_assert_irqs_enabled() to > local_irq_disable() so you would have it hidden behind lockdep with an > recursion check and everything. But this needs a lot of headers like > task_struct so… > I had once WARN_ON_ONCE(irqs_disabled()) added to testdrive it and had a > few false-positive in the early boot or constructs like in > __run_hrtimer(). I didn't look at it further… > >> Thanks, >> >> -corey > Sebastian
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