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

Ingo Molnar
2013-12-24T08:29:37Z

* Oleg Nesterov <oleg@redhat.com> wrote:

> 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.

You are right, today that's true technically because IIRC due to Sparc
quirks we happen to return 'flags' as a return value - still it's very
ugly and it could break anytime if we decide to do more aggressive
optimizations and actually directly save into 'flags'.

Note that even today there's a narrow exception: on UP we happen to
build it the other way around, so that we do:

local_irq_save(global_flags);
__acquire(lock);

This does not matter for any real code because on UP there is no
physical lock and __acquire() is empty code-wise, but any compiler
driven locking analysis tool using __attribute__ __context__(), if
built on UP, would see the unsafe locking pattern.

Thanks,

Ingo