Re: igc, macOS avoiding signals

[Helmut Eller]

On Mon, Dec 30 2024, Pip Cet wrote:

> "Helmut Eller" <eller.helmut@gmail.com> writes:
>> Very few of Emacs' signal handlers actually touch a barrier.  I've also
>
> Indeed.  These crashes are rare in typical usage, which doesn't mean we
> should delay fixing them until Emacs is "unstable enough".  It already
> is, IMHO, because we take that approach too frequently.
>
>> not seen any reproducable receipes for the "signal issues" that the igc
>> branch supposedly has.
>
> Removing the SIGPROF protection code should allow Ihor's recipe to crash
> again.

Talking about SIGPROF protection code.  It appears to me now (again)
that, for the SIGPROF handler, this pseudo code

  if (mps_arena_busy (<arena>))
    plog->gc_count = saturated_add (plog->gc_count, count);
  else
    record_backtrace (plog, count);

is safe.  If we don't hold the lock, then mps_arena_busy returns false
and we can access memory.  We are safe even if another thread has
claimed the lock by the time that we reach record_backtrace: the SIGPROF
handler will just block until the lock is released.

Does somebody disagree?

Helmut