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Date:	Wed, 2 Apr 2025 20:15:54 +0200
To:	cygwin AT cygwin DOT com
Subject:	Re: Crashes in cmake subprocesses since 3.6.0
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From:	Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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Cc:	Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
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Hi Takashi,

On Apr  3 01:52, Takashi Yano via Cygwin wrote:
> > Currently, I am looking into this problem.
> > 
> > What I noticed so far is:
> > * The problem occurs after the commit 7ed9adb356df.
> > * This problem is happen when fhandler_fifo_pipe::raw_write() returns
> >   error because cygwait(pipe_mtx, timeout) returns WAIT_FAILED. This seems
> >   to happen due to invalid _cygtls::signal_arrived handle for some reason.
> > * The following patch solves the issue.
> > 
> > diff --git a/winsup/cygwin/local_includes/cygtls.h b/winsup/cygwin/local_includes/cygtls.h
> > index f67e9136c..82a34aeca 100644
> > --- a/winsup/cygwin/local_includes/cygtls.h
> > +++ b/winsup/cygwin/local_includes/cygtls.h
> > @@ -228,6 +228,9 @@ public: /* Do NOT remove this public: line, it's a marker for gentls_offsets. */
> >    bool locked ();
> >    HANDLE get_signal_arrived (bool wait_for_lock = true)
> >    {
> > +    DWORD dummy;
> > +    if (signal_arrived && !GetHandleInformation (signal_arrived, &dummy))
> > +      signal_arrived = NULL;
> >      if (!signal_arrived)
> >        {
> >  	if (wait_for_lock)
> > 
> > Of course, this is not the right thing to do, but this clarifies that the
> > cause is _cygtis::signal_arrived being invalid even though it is not NULL.
> > The reason is not quite sure to me.
> > 
> > Any idea?
> 
> The following patch also can solve the issue. The problem seems
> to be related to fork().

So the invalid signal_arrived occurs in the child?

> Perhaps, the timming of calling _cygtls::fixup_after_fork(), that
> clears signal_arrived to NULL, might not be appropriate?

_cygtls::fixup_after_fork() is called in the middle of fork in the
child.  No other thread should be running in the child at the time.
How's it possible that a raw_write is running?

> diff --git a/winsup/cygwin/fork.cc b/winsup/cygwin/fork.cc
> index 0742ab363..793521314 100644
> --- a/winsup/cygwin/fork.cc
> +++ b/winsup/cygwin/fork.cc
> @@ -446,10 +446,14 @@ frok::parent (volatile char * volatile stack_here)
>        impure_beg = _impure_ptr;
>        impure_end = _impure_ptr + 1;
>      }
> +  HANDLE signal_arrived_back;
> +  signal_arrived_back = _my_tls.signal_arrived;
> +  _my_tls.signal_arrived = NULL;
>    rc = child_copy (hchild, true, !*with_forkables,
>  		   "stack", stack_here, ch.stackbase,
>  		   impure, impure_beg, impure_end,
>  		   NULL);
> +  _my_tls.signal_arrived = signal_arrived_back;

Weird.  But if that helps, wouldn't it make sense to keep
_my_tls.signal_arrived at the same value in the parent (signal handling
shouldn't run anyway at that time) and just set _my_tls.signal_arrived
in the child to NULL after child_copy()?

I.e.

    rc = child_copy (...);
    WriteProcessMemory (hchild, (PVOID) &_my_tls.signal_arrived,
			&null_ptr, sizeof null_ptr, NULL);

Still, I wonder in which thread raw_write is running during fork().


Corinna

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