try to simplify the locking code around busy maps.

vm_maps have a "feature" where they can mark that they're being
operated on by a specific proc, and then release the rwlock protecting
their state. to relock, you have to be the same proc that marked
it busy.

this diff tries to simplify it a bit. it basically has threads check
the busy field up front and rechecks the busy field inside the
rwlock. if you can sleep, it will sleep up front for the busy field
to become clear, rather than sleep on either the busy field or the
rwlock. some code paths clear the busy field without holding the
rwlock, so it doesn't make sense to me to be waiting for the busy
field but sleeping somewhere else.

ok claudio@ mpi@

2 files changed, 57 insertions, 60 deletions

diff --git a/sys/uvm/uvm_map.c b/sys/uvm/uvm_map.c
index 5fbcfe5f6e3..9de86fa2a7e 100644
--- a/sys/uvm/uvm_map.c
+++ b/sys/uvm/uvm_map.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: uvm_map.c,v 1.330 2024/07/24 12:17:31 mpi Exp $	*/
+/*	$OpenBSD: uvm_map.c,v 1.331 2024/10/20 11:28:17 dlg Exp $	*/
 /*	$NetBSD: uvm_map.c,v 1.86 2000/11/27 08:40:03 chs Exp $	*/
 
 /*
@@ -5201,68 +5201,77 @@ out:
 boolean_t
 vm_map_lock_try_ln(struct vm_map *map, char *file, int line)
 {
-	boolean_t rv;
+	int rv;
 
 	if (map->flags & VM_MAP_INTRSAFE) {
-		rv = mtx_enter_try(&map->mtx);
+		if (!mtx_enter_try(&map->mtx))
+			return FALSE;
 	} else {
+		struct proc *busy;
+
 		mtx_enter(&map->flags_lock);
-		if ((map->flags & VM_MAP_BUSY) && (map->busy != curproc)) {
-			mtx_leave(&map->flags_lock);
-			return (FALSE);
-		}
+		busy = map->busy;
 		mtx_leave(&map->flags_lock);
-		rv = (rw_enter(&map->lock, RW_WRITE|RW_NOSLEEP) == 0);
-		/* check if the lock is busy and back out if we won the race */
-		if (rv) {
-			mtx_enter(&map->flags_lock);
-			if ((map->flags & VM_MAP_BUSY) &&
-			    (map->busy != curproc)) {
-				rw_exit(&map->lock);
-				rv = FALSE;
-			}
-			mtx_leave(&map->flags_lock);
+		if (busy != NULL && busy != curproc)
+			return FALSE;
+
+		rv = rw_enter(&map->lock, RW_WRITE|RW_NOSLEEP);
+		if (rv != 0)
+			return FALSE;
+
+		/* to be sure, to be sure */
+		mtx_enter(&map->flags_lock);
+		busy = map->busy;
+		mtx_leave(&map->flags_lock);
+		if (busy != NULL && busy != curproc) {
+			rw_exit(&map->lock);
+			return FALSE;
 		}
 	}
 
-	if (rv) {
-		map->timestamp++;
-		LPRINTF(("map   lock: %p (at %s %d)\n", map, file, line));
-		uvm_tree_sanity(map, file, line);
-		uvm_tree_size_chk(map, file, line);
-	}
+	map->timestamp++;
+	LPRINTF(("map   lock: %p (at %s %d)\n", map, file, line));
+	uvm_tree_sanity(map, file, line);
+	uvm_tree_size_chk(map, file, line);
 
-	return (rv);
+	return TRUE;
 }
 
 void
 vm_map_lock_ln(struct vm_map *map, char *file, int line)
 {
 	if ((map->flags & VM_MAP_INTRSAFE) == 0) {
-		do {
-			mtx_enter(&map->flags_lock);
-tryagain:
-			while ((map->flags & VM_MAP_BUSY) &&
-			    (map->busy != curproc)) {
-				map->flags |= VM_MAP_WANTLOCK;
-				msleep_nsec(&map->flags, &map->flags_lock,
+		mtx_enter(&map->flags_lock);
+		for (;;) {
+			while (map->busy != NULL && map->busy != curproc) {
+				map->nbusy++;
+				msleep_nsec(&map->busy, &map->mtx,
 				    PVM, vmmapbsy, INFSLP);
+				map->nbusy--;
 			}
 			mtx_leave(&map->flags_lock);
-		} while (rw_enter(&map->lock, RW_WRITE|RW_SLEEPFAIL) != 0);
-		/* check if the lock is busy and back out if we won the race */
-		mtx_enter(&map->flags_lock);
-		if ((map->flags & VM_MAP_BUSY) && (map->busy != curproc)) {
-			rw_exit(&map->lock);
-			goto tryagain;
+
+			rw_enter_write(&map->lock);
+
+			/* to be sure, to be sure */
+			mtx_enter(&map->flags_lock);
+			if (map->busy != NULL && map->busy != curproc) {
+				/* go around again */
+				rw_exit_write(&map->lock);
+			} else {
+				/* we won */
+				break;
+			}
 		}
 		mtx_leave(&map->flags_lock);
 	} else {
 		mtx_enter(&map->mtx);
 	}
 
-	if (map->busy != curproc)
+	if (map->busy != curproc) {
+		KASSERT(map->busy == NULL);
 		map->timestamp++;
+	}
 	LPRINTF(("map   lock: %p (at %s %d)\n", map, file, line));
 	uvm_tree_sanity(map, file, line);
 	uvm_tree_size_chk(map, file, line);
@@ -5314,25 +5323,24 @@ vm_map_busy_ln(struct vm_map *map, char *file, int line)
 
 	mtx_enter(&map->flags_lock);
 	map->busy = curproc;
-	map->flags |= VM_MAP_BUSY;
 	mtx_leave(&map->flags_lock);
 }
 
 void
 vm_map_unbusy_ln(struct vm_map *map, char *file, int line)
 {
-	int oflags;
+	unsigned int nbusy;
 
 	KASSERT((map->flags & VM_MAP_INTRSAFE) == 0);
 	KASSERT(map->busy == curproc);
 
 	mtx_enter(&map->flags_lock);
-	oflags = map->flags;
+	nbusy = map->nbusy;
 	map->busy = NULL;
-	map->flags &= ~(VM_MAP_BUSY|VM_MAP_WANTLOCK);
 	mtx_leave(&map->flags_lock);
-	if (oflags & VM_MAP_WANTLOCK)
-		wakeup(&map->flags);
+
+	if (nbusy > 0)
+		wakeup(&map->busy);
 }
 
 void
diff --git a/sys/uvm/uvm_map.h b/sys/uvm/uvm_map.h
index 3a47d6dd899..9f8775f74ea 100644
--- a/sys/uvm/uvm_map.h
+++ b/sys/uvm/uvm_map.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: uvm_map.h,v 1.90 2024/06/18 12:37:29 jsg Exp $	*/
+/*	$OpenBSD: uvm_map.h,v 1.91 2024/10/20 11:28:17 dlg Exp $	*/
 /*	$NetBSD: uvm_map.h,v 1.24 2001/02/18 21:19:08 chs Exp $	*/
 
 /*
@@ -214,17 +214,6 @@ RBT_PROTOTYPE(uvm_map_addr, vm_map_entry, daddrs.addr_entry,
  *					map is write-locked.  may be tested
  *					without asserting `flags_lock'.
  *
- *		VM_MAP_BUSY		r/w; may only be set when map is
- *					write-locked, may only be cleared by
- *					thread which set it, map read-locked
- *					or write-locked.  must be tested
- *					while `flags_lock' is asserted.
- *
- *		VM_MAP_WANTLOCK		r/w; may only be set when the map
- *					is busy, and thread is attempting
- *					to write-lock.  must be tested
- *					while `flags_lock' is asserted.
- *
  *		VM_MAP_GUARDPAGES	r/o; must be specified at map
  *					initialization time.
  *					If set, guards will appear between
@@ -257,6 +246,7 @@ RBT_PROTOTYPE(uvm_map_addr, vm_map_entry, daddrs.addr_entry,
  *	a	atomic operations
  *	I	immutable after creation or exec(2)
  *	v	`vm_map_lock' (this map `lock' or `mtx')
+ *	f	flags_lock
  */
 struct vm_map {
 	struct pmap		*pmap;		/* [I] Physical map */
@@ -266,9 +256,10 @@ struct vm_map {
 
 	vsize_t			size;		/* virtual size */
 	int			ref_count;	/* [a] Reference count */
-	int			flags;		/* flags */
+	int			flags;		/* [f] flags */
 	unsigned int		timestamp;	/* Version number */
-	struct proc		*busy;		/* [v] thread holding map busy*/
+	struct proc		*busy;		/* [f] thread holding map busy*/
+	unsigned int		nbusy;		/* [f] waiters for busy */
 
 	vaddr_t			min_offset;	/* [I] First address in map. */
 	vaddr_t			max_offset;	/* [I] Last address in map. */
@@ -323,8 +314,6 @@ struct vm_map {
 #define	VM_MAP_PAGEABLE		0x01		/* ro: entries are pageable */
 #define	VM_MAP_INTRSAFE		0x02		/* ro: interrupt safe map */
 #define	VM_MAP_WIREFUTURE	0x04		/* rw: wire future mappings */
-#define	VM_MAP_BUSY		0x08		/* rw: map is busy */
-#define	VM_MAP_WANTLOCK		0x10		/* rw: want to write-lock */
 #define	VM_MAP_GUARDPAGES	0x20		/* rw: add guard pgs to map */
 #define	VM_MAP_ISVMSPACE	0x40		/* ro: map is a vmspace */
 #define	VM_MAP_PINSYSCALL_ONCE	0x100		/* rw: pinsyscall done */