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

[Oleg Nesterov <oleg@redhat.com>]

On 02/13, Nicholas Mc Guire wrote:
>
> On Thu, 12 Feb 2015, Oleg Nesterov wrote:
>
> > Nicholas, sorry, I sent the patch but forgot to CC you.
> > See https://lkml.org/lkml/2015/2/12/587
> >
> > And please note that "completion" was specially designed to guarantee
> > that complete() can't play with this memory after wait_for_completion/etc
> > returns.
> >
>
> hmmm.... I guess that "falling out of context" can happen in a number of cases
> with completion - any of the timeout/interruptible variants e.g:
>
> 	void xxx(void)
> 	{
> 		struct completion c;
>
> 		init_completion(&c);
>
> 		expose_this_completion(&c);
>
> 		wait_for_completion_timeout(&c,A_FEW_JIFFIES);
> 	}
>
> and if the other side did not call complete() within A_FEW_JIFFIES then
> it would result in the same failure - I don't think the API can prevent
> this type of bug.

Yes sure, but in this case the user of wait_for_completion_timeout() should
blame itself, it is simply buggy.

> Tt has to be ensured by additional locking

Yes, but

> drivers/misc/tifm_7xx1.c:tifm_7xx1_resume() resolve this issue by resetting
> the completion to NULL and testing for !NULL before calling complete()
> with appropriate locking protection access.

I don't understand this code, I can be easily wrong. but at first glance it
doesn't need completion at all. Exactly because it relies on the additional
fm->lock. ->finish_me could be "task_struct *", the tifm_7xx1_resume() could
simply do schedule_timeout(), tifm_7xx1_isr() could do wake_up_process().
Nevermind, this is off-topic and most probably I misread this code.

> Never the less of course the proposed change in completion_done() was a bug -
> many thanks for catching that so quickly !

OK, perhaps you can ack the fix I sent?

Oleg.