Fix a lock reversal problem in the VM subsystem related to threaded

programs.   There is a case during a fork() which can cause a deadlock.

From Tor -
The workaround that consists of setting a flag in the vm map that
indicates that a fork is in progress and using that mark in the page
fault handling to force a revalidation failure.  That change will only
affect (pessimize) page fault handling during fork for threaded
(linuxthreads style) applications and applications using aio_*().

Submited by: tegge

sys/vm/vm_fault.c

@@ -725,6 +725,12 @@ RetryFault:;
 			vput(fs.vp);
 			fs.vp = NULL;
 		}
+		
+		if (fs.map->infork) {
+			release_page(&fs);
+			unlock_and_deallocate(&fs);
+			goto RetryFault;
+		}
 
 		/*
 		 * To avoid trying to write_lock the map while another process

sys/vm/vm_map.c

@@ -254,6 +254,7 @@ vm_map_init(map, min, max)
 	map->nentries = 0;
 	map->size = 0;
 	map->system_map = 0;
+	map->infork = 0;
 	map->min_offset = min;
 	map->max_offset = max;
 	map->first_free = &map->header;
@@ -2123,6 +2124,7 @@ vmspace_fork(vm1)
 	vm_object_t object;
 
 	vm_map_lock(old_map);
+	old_map->infork = 1;
 
 	vm2 = vmspace_alloc(old_map->min_offset, old_map->max_offset);
 	bcopy(&vm1->vm_startcopy, &vm2->vm_startcopy,
@@ -2215,6 +2217,7 @@ vmspace_fork(vm1)
 	}
 
 	new_map->size = old_map->size;
+	old_map->infork = 0;
 	vm_map_unlock(old_map);
 
 	return (vm2);

sys/vm/vm_map.h

@@ -162,6 +162,7 @@ struct vm_map {
 	int nentries;			/* Number of entries */
 	vm_size_t size;			/* virtual size */
 	u_char system_map;		/* Am I a system map? */
+	u_char infork;			/* Am I in fork processing? */
 	vm_map_entry_t hint;		/* hint for quick lookups */
 	unsigned int timestamp;		/* Version number */
 	vm_map_entry_t first_free;	/* First free space hint */