Re: BUG: spinlock bad magic on CPU#0, migration/0/9

[Oleg Nesterov <oleg@redhat.com>]

On 02/11, Davidlohr Bueso wrote:
>
> On Wed, 2015-02-11 at 16:34 -0800, Paul E. McKenney wrote:
> > Hello!
> >
> > Did an earlier-than-usual port of v3.21 patches to post-v3.19, and
> > hit the following on x86_64.  This happened after about 15 minutes of
> > rcutorture.  In contrast, I have been doing successful 15-hour runs
> > on v3.19.  I will check reproducibility and try to narrow it down.
> > Might this be a duplicate of the bug that Raghavendra posted a fix for?
> >
> > Anyway, this was on 3e8c04eb1174 (Merge branch 'for-3.20' of
> > git://git.kernel.org/pub/scm/linux/kernel/git/tj/libata).
> >
> > [  837.287011] BUG: spinlock bad magic on CPU#0, migration/0/9
> > [  837.287013]  lock: 0xffff88001ea0fe80, .magic: ffffffff, .owner:  gî<81>ÿÿÿÿ/0, .owner_cpu: -42
> > [  837.287013] CPU: 0 PID: 9 Comm: migration/0 Not tainted 3.19.0+ #1
> > [  837.287013] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> > [  837.287013]  ffff88001ea0fe80 ffff88001ea0bc78 ffffffff818f6f4b ffffffff810a5a51
> > [  837.287013]  ffffffff81e500e0 ffff88001ea0bc98 ffffffff818f3755 ffff88001ea0fe80
> > [  837.287013]  ffffffff81ca4396 ffff88001ea0bcb8 ffffffff818f377b ffff88001ea0fe80
> > [  837.287013] Call Trace:
> > [  837.287013]  [<ffffffff818f6f4b>] dump_stack+0x45/0x57
> > [  837.287013]  [<ffffffff810a5a51>] ? console_unlock+0x1f1/0x4c0
> > [  837.287013]  [<ffffffff818f3755>] spin_dump+0x8b/0x90
> > [  837.287013]  [<ffffffff818f377b>] spin_bug+0x21/0x26
> > [  837.287013]  [<ffffffff8109923c>] do_raw_spin_unlock+0x5c/0xa0
> > [  837.287013]  [<ffffffff81902587>] _raw_spin_unlock_irqrestore+0x27/0x50
> > [  837.287013]  [<ffffffff8108f0a1>] complete+0x41/0x50
>
> We did have some recent changes in completions:
>
> 7c34e318 (sched/completion: Add lock-free checking of the blocking case)
> de30ec47 (sched/completion: Remove unnecessary ->wait.lock serialization when reading completion state)
>
> The second one being more related (although both appear to make sense).
> Perhaps some subtle implication in the completion_done side that
> disappeared with the spinlock?

At first glance both changes look suspicious. Unless at least document how
you can use these helpers.

Consider this code:

	void xxx(void)
	{
		struct completion c;

		init_completion(&c);

		expose_this_completion(&c);

		while (!completion_done(&c)
			schedule_timeout_uninterruptible(1);
	}

Before that change this code was correct, now it is not. Hmm and note that
this is what stop_machine_from_inactive_cpu() does although I do not know
if this is related or not.

Because completion_done() can now race with complete(), the final
spin_unlock() can write to the memory after it was freed/reused. In this
case it can write to the stack after return.

Add CC's.

Oleg.