Re: [PATCH v6] taskstats: fix data-race

[Dmitry Vyukov <dvyukov@google.com>]

On Thu, Oct 24, 2019 at 3:05 PM Andrea Parri <parri.andrea@gmail.com> wrote:
>
> On Thu, Oct 24, 2019 at 01:51:20PM +0200, Dmitry Vyukov wrote:
> > On Thu, Oct 24, 2019 at 1:32 PM Andrea Parri <parri.andrea@gmail.com> wrote:
> > >
> > > > How these later loads can be completely independent of the pointer
> > > > value? They need to obtain the pointer value from somewhere. And this
> > > > can only be done by loaded it. And if a thread loads a pointer and
> > > > then dereferences that pointer, that's a data/address dependency and
> > > > we assume this is now covered by READ_ONCE.
> > >
> > > The "dependency" I was considering here is a dependency _between the
> > > load of sig->stats in taskstats_tgid_alloc() and the (program-order)
> > > later loads of *(sig->stats) in taskstats_exit().  Roughly speaking,
> > > such a dependency should correspond to a dependency chain at the asm
> > > or registers level from the first load to the later loads; e.g., in:
> > >
> > >   Thread [register r0 contains the address of sig->stats]
> > >
> > >   A: LOAD r1,[r0]       // LOAD_ACQUIRE sig->stats
> > >      ...
> > >   B: LOAD r2,[r0]       // LOAD *(sig->stats)
> > >   C: LOAD r3,[r2]
> > >
> > > there would be no such dependency from A to C.  Compare, e.g., with:
> > >
> > >   Thread [register r0 contains the address of sig->stats]
> > >
> > >   A: LOAD r1,[r0]       // LOAD_ACQUIRE sig->stats
> > >      ...
> > >   C: LOAD r3,[r1]       // LOAD *(sig->stats)
> > >
> > > AFAICT, there's no guarantee that the compilers will generate such a
> > > dependency from the code under discussion.
> >
> > Fixing this by making A ACQUIRE looks like somewhat weird code pattern
> > to me (though correct). B is what loads the address used to read
> > indirect data, so B ought to be ACQUIRE (or LOAD-DEPENDS which we get
> > from READ_ONCE).
> >
> > What you are suggesting is:
> >
> > addr = ptr.load(memory_order_acquire);
> > if (addr) {
> >   addr = ptr.load(memory_order_relaxed);
> >   data = *addr;
> > }
> >
> > whereas the canonical/non-convoluted form of this pattern is:
> >
> > addr = ptr.load(memory_order_consume);
> > if (addr)
> >   data = *addr;
>
> No, I'd rather be suggesting:
>
>   addr = ptr.load(memory_order_acquire);
>   if (addr)
>     data = *addr;
>
> since I'd not expect any form of encouragement to rely on "consume" or
> on "READ_ONCE() + true-address-dependency" from myself.  ;-)

But why? I think kernel contains lots of such cases and it seems to be
officially documented by the LKMM:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/explanation.txt
address dependencies and ppo
Am I missing something? Why do we need acquire with address dependency?

> IAC, v6 looks more like:
>
>   addr = ptr.load(memory_order_consume);
>   if (!!addr)
>     *ptr = 1;
>   data = *ptr;
>
> to me (hence my comments/questions ...).
>
> Thanks,
>   Andrea