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

[Dmitry Vyukov]

On Thu, Oct 24, 2019 at 1:32 PM Andrea Parri <parri.andrea@xxxxxxxxx> 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;

Moreover the second load of ptr is not even atomic in our case, so it
is a subject to another data race?