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

[Andrea Parri <parri.andrea@gmail.com>]

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.  ;-)

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