Re: [PATCH] async_die_is_recursing: fix use of pthread_getspecific for Fedora

From: Jeff King <peff@peff.net>
To: Carlo Arenas <carenas@gmail.com>
Cc: Victoria Dye via GitGitGadget <gitgitgadget@gmail.com>,
	git@vger.kernel.org, gitster@pobox.com, philipoakley@iee.email,
	eschwartz@archlinux.org, Victoria Dye <vdye@github.com>
Subject: Re: [PATCH] async_die_is_recursing: fix use of pthread_getspecific for Fedora
Date: Wed, 3 Nov 2021 22:37:35 -0400	[thread overview]
Message-ID: <YYNHb0qq2n5OWC+R@coredump.intra.peff.net> (raw)
In-Reply-To: <CAPUEspjw4zypiywCBRc=y9uC4G5CKTE35GLVf_wDY436oO0C5w@mail.gmail.com>

On Wed, Nov 03, 2021 at 07:23:23PM -0700, Carlo Arenas wrote:

> > diff --git a/run-command.c b/run-command.c
> > index 7ef5cc712a9..a82cf69e7d3 100644
> > --- a/run-command.c
> > +++ b/run-command.c
> > @@ -1099,7 +1099,7 @@ static NORETURN void die_async(const char *err, va_list params)
> >  static int async_die_is_recursing(void)
> >  {
> >         void *ret = pthread_getspecific(async_die_counter);
> > -       pthread_setspecific(async_die_counter, (void *)1);
> > +       pthread_setspecific(async_die_counter, &ret); /* set to any non-NULL valid pointer */
> 
> I guess this would work, since the pointer is never dereferenced, but
> the use of (void *)1 was hacky, and this warning seems like the right
> time to make it less so.
> 
> Would a dynamically allocated pthread_local variable be a better
> option,  or even a static global, since we don't care about its value
> so no need to worry about any races?

Yeah, I had the same thought. I think what's in the patch above is OK in
practice, but it sure _feels_ wrong to store the address of an auto
variable that goes out of scope.

I'm OK with it as a minimal fix, though, to get things unstuck. The
commit message nicely explains what's going on, and the original (which
it looks like blames to me ;) ) is pretty gross, too.

Keeping an actual counter variable would be the least-confusing thing,
IMHO, but that implies allocating per-thread storage (which means having
to clean it up). And we really only care about counting up to "1", so
the boolean "do we have a pointer" is fine. The static variable you
suggest might be a good middle ground there, and we could even use it
for the comparison to make things more clear.  Something like:

  static int async_die_is_recursing(void)
  {
	  static int async_recursing_flag;
          void *ret = pthread_getspecific(async_die_counter);
	  pthread_setspecific(async_die_counter, &async_recursing_flag);
	  return ret == &async_recursing_flag;
  }

But I don't feel that strongly either way.

-Peff