Re: spinlock_irqsave() && flags (Was: pm80xx: Spinlock fix)

Oleg Nesterov
2013-12-23T18:33:31Z

On 12/23, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@redhat.com> wrote:
>
> > Initially I thought that this is obviously wrong, irqsave/irqrestore
> > assume that "flags" is owned by the caller, not by the lock. And
> > iirc this was certainly wrong in the past.
> >
> > But when I look at spinlock.c it seems that this code can actually
> > work. _irqsave() writes to FLAGS after it takes the lock, and
> > _irqrestore() has a copy of FLAGS before it drops this lock.
>
> I don't think that's true: if it was then the lock would not be
> irqsave, a hardware-irq could come in after the lock has been taken
> and before flags are saved+disabled.

I do agree that this pattern is not safe, that is why I decided to ask.

But, unless I missed something, with the current implementation
spin_lock_irqsave(lock, global_flags) does:

unsigned long local_flags;

local_irq_save(local_flags);
spin_lock(lock);

global_flags = local_flags;

so the access to global_flags is actually serialized by lock.

> So AFAICS this is an unsafe pattern, beyond being ugly as hell.

Yes, I think the same.

Oleg.