replace the flockfile backend with a per FILE recursive mutex.

the flockfile implementation in thread/rthread_file.c used an
external lock, and associated it with the relevant FILE * as needed.
this isn't great for a lot of reasons, complexity being the big
one, but the straw that broke the camels back is that it uses a
single spinlock to coordinate all of this, which in turn generates
a lot of sched_yield syscalls.

this avoids all the code complexity and the spinlock by just embedding
a small __rctmx in every FILE.

tested by and ok tb@ jca@
ok claudio@

lib/libc/hidden/_stdio.h

···
-/*	$OpenBSD: _stdio.h,v 1.2 2025/07/23 08:17:23 yasuoka Exp $	*/
+/*	$OpenBSD: _stdio.h,v 1.3 2025/08/04 01:44:32 dlg Exp $	*/
 /*	$NetBSD: stdio.h,v 1.18 1996/04/25 18:29:21 jtc Exp $	*/
 
 /*-
···
 
 #ifndef _LIBC__STDIO_H_
 #define _LIBC__STDIO_H_
+
+#include "thread_private.h"
 
 /*
  * NB: to fit things in six character monocase externals, the stdio

lib/libc/hidden/stdio.h

···
-/*	$OpenBSD: stdio.h,v 1.9 2025/07/16 15:33:05 yasuoka Exp $	*/
+/*	$OpenBSD: stdio.h,v 1.10 2025/08/04 01:44:32 dlg Exp $	*/
 /*
  * Copyright (c) 2015 Philip Guenther <[e-mail]>
  *
···
 
 __BEGIN_HIDDEN_DECLS
 int	__cleanfile(FILE *, int _doclose);
+void	__relefile(FILE *);
 char	*_mktemp(char *);
 __END_HIDDEN_DECLS
 

lib/libc/include/thread_private.h

···
-/* $OpenBSD: thread_private.h,v 1.39 2025/07/16 16:22:58 deraadt Exp $ */
+/* $OpenBSD: thread_private.h,v 1.40 2025/08/04 01:44:33 dlg Exp $ */
 
 /* PUBLIC DOMAIN: No Rights Reserved. Marco S Hyman <[e-mail]> */
 
···
 	struct __cmtx		mtx;
 	unsigned int		depth;
 };
+
+#define __RCMTX_INITIALIZER() {						\
+	.owner = NULL,							\
+	.mtx = __CMTX_INITIALIZER(),					\
+	.depth = 0,							\
+}
 
 struct pthread_mutex_attr {
 	int ma_type;

lib/libc/stdio/fclose.c

···
-/*	$OpenBSD: fclose.c,v 1.14 2025/07/16 15:33:05 yasuoka Exp $ */
+/*	$OpenBSD: fclose.c,v 1.15 2025/08/04 01:44:33 dlg Exp $ */
 /*-
  * Copyright (c) 1990, 1993 The Regents of the University of California.
  * Copyright (c) 2013 Mariusz Zaborski <[e-mail]>
···
 	if (HASLB(fp))
 		FREELB(fp);
 	fp->_r = fp->_w = 0;	/* Mess up if reaccessed. */
+	return r;
+}
+
+void
+__relefile(FILE *fp)
+{
 	fp->_flags = 0;		/* Release this FILE for reuse. */
-	return r;
 }
 
 int
···
 	FLOCKFILE(fp);
 	r = __cleanfile(fp, 1);
 	FUNLOCKFILE(fp);
+	__relefile(fp);
 	return (r);
 }
 DEF_STRONG(fclose);

lib/libc/stdio/fdclose.c

···
-/*	$OpenBSD: fdclose.c,v 1.1 2025/07/16 15:33:05 yasuoka Exp $ */
+/*	$OpenBSD: fdclose.c,v 1.2 2025/08/04 01:44:33 dlg Exp $ */
 /*-
  * Copyright (c) 1990, 1993 The Regents of the University of California.
  * Copyright (c) 2013 Mariusz Zaborski <[e-mail]>
···
 		r = __cleanfile(fp, 0);
 	}
 	FUNLOCKFILE(fp);
+	__relefile(fp);
 
 	return r;
 }

lib/libc/stdio/fileext.h

···
-/*	$OpenBSD: fileext.h,v 1.2 2005/06/17 20:40:32 espie Exp $	*/
+/*	$OpenBSD: fileext.h,v 1.3 2025/08/04 01:44:33 dlg Exp $	*/
 /* $NetBSD: fileext.h,v 1.5 2003/07/18 21:46:41 nathanw Exp $ */
 
 /*-
···
 struct __sfileext {
 	struct	__sbuf _ub; /* ungetc buffer */
 	struct wchar_io_data _wcio;	/* wide char io status */
+	struct __rcmtx _lock;
 };
 
 #define _EXT(fp) ((struct __sfileext *)((fp)->_ext._base))
···
 	_UB(fp)._base = NULL; \
 	_UB(fp)._size = 0; \
 	WCIO_INIT(fp); \
+	__rcmtx_init(&_EXT(fp)->_lock); \
 } while (0)
 
 #define _FILEEXT_SETUP(f, fext) \

lib/libc/stdio/findfp.c

···
-/*	$OpenBSD: findfp.c,v 1.21 2025/07/16 15:33:05 yasuoka Exp $ */
+/*	$OpenBSD: findfp.c,v 1.22 2025/08/04 01:44:33 dlg Exp $ */
 /*-
  * Copyright (c) 1990, 1993
  *	The Regents of the University of California.  All rights reserved.
···
 static struct glue *lastglue = &uglue;
 static void *sfp_mutex;
 
-static struct __sfileext __sFext[3];
+static struct __sfileext __sFext[3] = {
+	{ ._lock = __RCMTX_INITIALIZER() },
+	{ ._lock = __RCMTX_INITIALIZER() },
+	{ ._lock = __RCMTX_INITIALIZER() },
+};
+
 /*
  * These are separate variables because they may end up copied
  * into program images via COPY relocations, so their addresses

lib/libc/stdio/flockfile.c

···
-/*	$OpenBSD: flockfile.c,v 1.9 2016/05/07 19:05:22 guenther Exp $	*/
+/*	$OpenBSD: flockfile.c,v 1.10 2025/08/04 01:44:33 dlg Exp $	*/
 
 #include <stdio.h>
 #include "local.h"
···
 int
 ftrylockfile(FILE *fp)
 {
-	if (_thread_cb.tc_ftrylockfile != NULL)
-		return (_thread_cb.tc_ftrylockfile(fp));
+	if (__isthreaded)
+		return __rcmtx_enter_try(&_EXT(fp)->_lock) ? 0 : 1;
 
 	return 0;
 }

lib/libc/stdio/local.h

···
-/*	$OpenBSD: local.h,v 1.25 2016/05/23 00:21:48 guenther Exp $	*/
+/*	$OpenBSD: local.h,v 1.26 2025/08/04 01:44:33 dlg Exp $	*/
 
 /*-
  * Copyright (c) 1990, 1993
···
 
 #define FLOCKFILE(fp)							\
 	do {								\
-		if (_thread_cb.tc_flockfile != NULL)			\
-			_thread_cb.tc_flockfile(fp);			\
+		if (__isthreaded)					\
+			__rcmtx_enter(&_EXT(fp)->_lock);		\
 	} while (0)
 #define FUNLOCKFILE(fp)							\
 	do {								\
-		if (_thread_cb.tc_funlockfile != NULL)			\
-			_thread_cb.tc_funlockfile(fp);			\
+		if (__isthreaded)					\
+			__rcmtx_leave(&_EXT(fp)->_lock);		\
 	} while (0)

lib/libc/thread/Makefile.inc

···
-#	$OpenBSD: Makefile.inc,v 1.19 2020/02/06 03:13:45 jsg Exp $
+#	$OpenBSD: Makefile.inc,v 1.20 2025/08/04 01:44:33 dlg Exp $
 
 .PATH: ${LIBCSRCDIR}/thread
 
···
 SRCS+=	rthread.c \
 	rthread_condattr.c \
 	rthread_debug.c \
-	rthread_file.c \
 	rthread_libc.c \
 	rthread_once.c \
 	rthread_tls.c \

lib/libc/thread/callbacks.c

···
 		 * here when we actually need to prep for doing MT.
 		 */
 		_thread_cb.tc_canceled		= _thread_canceled;
-		_thread_cb.tc_flockfile		= _thread_flockfile;
-		_thread_cb.tc_ftrylockfile	= _thread_ftrylockfile;
-		_thread_cb.tc_funlockfile	= _thread_funlockfile;
 		_thread_cb.tc_malloc_lock	= _thread_malloc_lock;
 		_thread_cb.tc_malloc_unlock	= _thread_malloc_unlock;
 		_thread_cb.tc_atexit_lock	= _thread_atexit_lock;