Index: head/lib/libthr/arch/i386/i386/_setcurthread.c
===================================================================
--- head/lib/libthr/arch/i386/i386/_setcurthread.c	(revision 114772)
+++ head/lib/libthr/arch/i386/i386/_setcurthread.c	(revision 114773)
@@ -1,126 +1,129 @@
 /*
  * Copyright (c) 2003, Jeffrey Roberson <jeff@freebsd.org>
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice unmodified, this list of conditions, and the following
  *    disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  * $FreeBSD$
  */
 
 #include <sys/types.h>
 #include <sys/ucontext.h>
 
 #include <stdio.h>
 
 #include <machine/sysarch.h>
 #include <machine/segments.h>
 
 #define	MAXTHR	128
 
 #define	LDT_INDEX(x)	(((long)(x) - (long)ldt_entries) / sizeof(ldt_entries[0]))
 
 void **ldt_free = NULL;
 static int ldt_inited = 0;
 void *ldt_entries[MAXTHR];
 
 static void ldt_init(void);
 
 static void
 ldt_init(void)
 {
 	int i;
 
 	ldt_free = &ldt_entries[NLDT];
 
 	for (i = 0; i < MAXTHR - 1; i++)
 		ldt_entries[i] = (void *)&ldt_entries[i + 1];
 
 	ldt_entries[MAXTHR - 1] = NULL;
 
 	ldt_inited = 1;
 }
 
 void
 _retire_thread(void *entry)
 {
-	*(void **)entry = *ldt_free;
+	if (ldt_free == NULL)
+		*(void **)entry = NULL;
+	else
+		*(void **)entry = *ldt_free;
 	ldt_free = entry;
 }
 
 void *
 _set_curthread(ucontext_t *uc, void *thr)
 {
 	union descriptor desc;
 	void **ldt_entry;
 	int ldt_index;
 	int error;
 
 	if (ldt_inited == NULL)
 		ldt_init();
 
 	if (ldt_free == NULL)
 		abort();
 
 	/*
 	 * Pull one off of the free list and update the free list pointer.
 	 */
 	ldt_entry = ldt_free;
 	ldt_free = (void **)*ldt_entry;
 
 	/*
 	 * Cache the address of the thread structure here.  This is
 	 * what the gs register will point to.
 	 */
 	*ldt_entry = thr;
 	ldt_index = LDT_INDEX(ldt_entry);
 
 	bzero(&desc, sizeof(desc));
 
 	/*
 	 * Set up the descriptor to point into the ldt table which contains
 	 * only a pointer to the thread.
 	 */
 	desc.sd.sd_lolimit = sizeof(*ldt_entry);
 	desc.sd.sd_lobase = (unsigned int)ldt_entry & 0xFFFFFF;
 	desc.sd.sd_type = SDT_MEMRO;
 	desc.sd.sd_dpl = SEL_UPL;
 	desc.sd.sd_p = 1;
 	desc.sd.sd_hilimit = 0;
 	desc.sd.sd_xx = 0;
 	desc.sd.sd_def32 = 1;
 	desc.sd.sd_gran = 0;
 	desc.sd.sd_hibase = (unsigned int)ldt_entry >> 24;
 
 	error = i386_set_ldt(ldt_index, &desc, 1);
 	if (error == -1)
 		abort(); 
 
 	/*
 	 * Set up our gs with the index into the ldt for this entry.
 	 */
 	if (uc != NULL)
 		uc->uc_mcontext.mc_gs = LSEL(ldt_index, SEL_UPL);
 	else
 		_set_gs(LSEL(ldt_index, SEL_UPL));
 
 	return (ldt_entry);
 }