Message-ID: <6b839441-cee1-aefc-e432-7a0473f7da61@mvista.com>
Date:   Tue, 6 Mar 2018 16:51:53 -0600
From:   Corey Minyard <cminyard@...sta.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-rt-users <linux-rt-users@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kent Overstreet <koverstreet@...gle.com>
Subject: Re: Warning from swake_up_all in 4.14.15-rt13 non-RT

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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