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Date: Thu, 3 Apr 2025 12:32:35 +0900
To: cygwin AT cygwin DOT com
Subject: Re: Crashes in cmake subprocesses since 3.6.0
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From: Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
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On Wed, 2 Apr 2025 20:15:54 +0200
Corinna Vinschen wrote:
> 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?

Yes.

> > 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);

This does not work (ERROR_PARTIAL_COPY), however, the following
works.

diff --git a/winsup/cygwin/fork.cc b/winsup/cygwin/fork.cc
index 0742ab363..e11a7d507 100644
--- a/winsup/cygwin/fork.cc
+++ b/winsup/cygwin/fork.cc
@@ -450,10 +450,15 @@ frok::parent (volatile char * volatile stack_here)
 		   "stack", stack_here, ch.stackbase,
 		   impure, impure_beg, impure_end,
 		   NULL);
+  SIZE_T nb;
+  _cygtls tls1;
+  tls1 = _my_tls;
+  tls1.signal_arrived = NULL;
+  WriteProcessMemory (hchild, (PVOID) &_my_tls, &tls1, sizeof (tls1), &nb);
 
   __malloc_unlock ();
   locked = false;
-  if (!rc)
+  if (!rc || nb != sizeof (tls1))
     {
       this_errno = get_errno ();
       error ("pid %u, exitval %p", pi.dwProcessId, ch.exit_code);

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

Weird enough, raw_write() is called in the main thread (_main_tls).
Any chance, fixup_after_fork() is not called?

-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

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