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Date: Sun, 2 Jun 2024 23:27:17 +0900
To: Bruno Haible <bruno@clisp.org>
Cc: cygwin@cygwin.com
Subject: Re: [PATCH v2] Cygwin: pthread: Fix a race issue introduced by the
 commit 2c5433e5da82
Message-Id: <20240602232717.25baf1bdfe7ac283dea96899@nifty.ne.jp>
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From: Takashi Yano via Cygwin <cygwin@cygwin.com>
Reply-To: Takashi Yano <takashi.yano@nifty.ne.jp>
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On Sun, 02 Jun 2024 15:14:51 +0200
Bruno Haible wrote:
> Hi Takashi Yano,
> 
> > The result is as follows (submitted as v4 patch).
> > 
> > int
> > pthread::once (pthread_once_t *once_control, void (*init_routine) (void))
> > {
> >   /* Sign bit of once_control->state is used as done flag.
> >      Similary, the next significant bit is used as destroyed flag. */
> >   const int done = INT_MIN;		/* 0b1000000000000000 */
> >   const int destroyed = INT_MIN >> 1;	/* 0b1100000000000000 */
> >   if (once_control->state & done)
> >     return 0;
> > 
> >   /* The type of &once_control->state is int *, which is compatible with
> >      LONG * (the type of the pointer argument of InterlockedXxx()). */
> >   if ((InterlockedIncrement (&once_control->state) & done) == 0)
> >     {
> >       pthread_mutex_lock (&once_control->mutex);
> >       if (!(once_control->state & done))
> > 	{
> > 	  init_routine ();
> > 	  InterlockedOr (&once_control->state, done);
> > 	}
> >       pthread_mutex_unlock (&once_control->mutex);
> >     }
> >   InterlockedDecrement (&once_control->state);
> >   if (InterlockedCompareExchange (&once_control->state,
> > 				  destroyed, done) == done)
> >     pthread_mutex_destroy (&once_control->mutex);
> >   return 0;
> > }
> > ...
> > I believe both codes are equivalent. Could you please check?
> 
> Yes, they are equivalent. This code is free of race conditions. (Let's
> hope I am not making a mistake again.)
> 
> For legibility I would write the constant values as bit masks:
>   0x80000000
>   0xc0000000
> and - following the habit that constant integers should have names in
> upper case - I would rename
>   done → DONE
>   destroyed → DESTROYED

Thanks for checking. I'll push the patch after the modification.

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>

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