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Date:	Wed, 16 Jul 2025 16:44:34 +0200
To:	cygwin AT cygwin DOT com
Subject:	Re: doxygen hangs when many call graphs are generated.
Message-ID:	<aHe60g28H_pTjVux@calimero.vinschen.de>
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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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On Jul 16 22:45, Takashi Yano via Cygwin wrote:
> On Wed, 16 Jul 2025 21:36:04 +0900
> Takashi Yano via Cygwin <cygwin AT cygwin DOT com> wrote:
> > On Wed, 16 Jul 2025 21:13:59 +0900
> > Takashi Yano via Cygwin <cygwin AT cygwin DOT com> wrote:
> > 
> > > On Tue, 17 Jun 2025 21:46:47 +0900
> > > Takashi Yano wrote:
> > > > I encountered a problem of doxygen when many call graphs are generated.
> > > > 
> > > > How to reproduce:
> > > > 1) Make a empty directory.
> > > > 2) Place two files (Doxyfile, x.c) attached in the directory.
> > > > 3) Run doxygen in the directory.
> > > > 
> > > > It seems that this is a bug of cygwin1.dll and the cause is
> > > > a deadlock in newlib/libc/stdio/findfp.c:__fp_lock_all().
> 
> A simple reproducer is as follows.
> 
> #include <unistd.h>
> #include <stdio.h>
> #include <pthread.h>
> #include <sys/wait.h>
> 
> void *func(void *arg)
> {
> 	while (1) {
> 		FILE *f = fopen("/usr/bin/cygwin1.dll", "r"); /* Open something */
> 		fclose(f);
> 	}
> 	return NULL;
> }
> 
> int main()
> {
> 	pthread_t th;
> 	pthread_create(&th, NULL, func, NULL);
> 	int cnt = 0;
> 	while (1) {
> 		if (fork() == 0) {
> 			return 0;
> 		} else {
> 			wait(NULL);
> 			printf("Done %d\n", cnt++);
> 		}
> 	}
> }

It's a deadlock on __sfp_recursive_mutex.  fclose() locks the
file via _flockfile() and then calls __sfp_lock_acquire() before
calling _funlockfile(), while __fp_lock_all() called from fork()
first calls __sfp_lock_acquire() and then tries to lock all files
via _flockfile().

It's really a classic case of dumb locking in newlib.

No idea how to fix this yet, though...


Corinna

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