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

On 02/12, Oleg Nesterov wrote:
>
> On 02/12, Nicholas Mc Guire wrote:
> >
> > On Thu, 12 Feb 2015, Oleg Nesterov wrote:
> >
> > > No, sorry, only the 2nd one.
> > >
> > > > 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);
> >
> > But that would not break due to the change - even if completion_done() had a
> > problem - complete_done() is not consuming x->done it is only checking it?
>
> Nicholas, looks like you didn't read the text below:
>
> > > > 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.
>
> Or I misunderstood you.
>
> > > bool completion_done(struct completion *x)
> > > {
> > > - return !!ACCESS_ONCE(x->done);
> > > + if (!READ_ONCE(x->done))
> > > + return false;
> > > +
> > > + smp_rmb();
> > > + spin_unlock_wait(&x->wait.lock);
> > > + return true;
> >
> > what would be the sense of the spin_unlock_wait here ?
> > all you are interested in here is the state of x->done
>
> No. Please see above.
>
> > regarding the smp_rmb() where would the counterpart to that be ?
>
> to avoid the reordering, we should not read ->wait.lock before ->done.
>
> Oleg.



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