kernel: avoid palloc lock during mmurelease()

Previously, mmurelease() was always called with
palloc spinlock held.

This is unneccesary for some mmurelease()
implementations as they wont release pages
to the palloc pool.

This change removes pagechainhead() and
pagechaindone() and replaces them with just
freepages() call, which aquires the palloc
lock internally as needed.

freepages() avoids holding the palloc lock
while walking the linked list of pages,
avoding some lock contention.

1 files changed, 15 insertions, 21 deletions

diff --git a/sys/src/9/pc/mmu.c b/sys/src/9/pc/mmu.c
index df0c23520..a21c94197 100644
--- a/sys/src/9/pc/mmu.c
+++ b/sys/src/9/pc/mmu.c
@@ -320,54 +320,48 @@ mmuswitch(Proc* proc)
  *   cleaning any user entries in the pdb (proc->mmupdb);
  *   if there's a pdb put it in the cache of pre-initialised pdb's
  *   for this processor (m->pdbpool) or on the process' free list;
- *   finally, place any pages freed back into the free pool (palloc).
- * This routine is only called from schedinit() with palloc locked.
+ *   finally, place any pages freed back into the free pool (freepages).
  */
 void
 mmurelease(Proc* proc)
 {
-	Page *page, *next;
 	ulong *pdb;
+	Page *page;
 
-	if(islo())
-		panic("mmurelease: islo");
 	taskswitch(PADDR(m->pdb), (ulong)m + BY2PG);
-	if(proc->kmaptable != nil){
+	if((page = proc->kmaptable) != nil){
+		if(page->ref != 1)
+			panic("mmurelease: kmap ref %ld", page->ref);
 		if(proc->mmupdb == nil)
 			panic("mmurelease: no mmupdb");
-		if(--proc->kmaptable->ref != 0)
-			panic("mmurelease: kmap ref %ld", proc->kmaptable->ref);
 		if(proc->nkmap)
 			panic("mmurelease: nkmap %d", proc->nkmap);
 		/*
 		 * remove kmaptable from pdb before putting pdb up for reuse.
 		 */
 		pdb = tmpmap(proc->mmupdb);
-		if(PPN(pdb[PDX(KMAP)]) != proc->kmaptable->pa)
+		if(PPN(pdb[PDX(KMAP)]) != page->pa)
 			panic("mmurelease: bad kmap pde %#.8lux kmap %#.8lux",
-				pdb[PDX(KMAP)], proc->kmaptable->pa);
+				pdb[PDX(KMAP)], page->pa);
 		pdb[PDX(KMAP)] = 0;
 		tmpunmap(pdb);
+
 		/*
 		 * move kmaptable to free list.
 		 */
-		pagechainhead(proc->kmaptable);
+		page->next = proc->mmufree;
+		proc->mmufree = page;
 		proc->kmaptable = nil;
 	}
-	if(proc->mmupdb != nil){
+	if((page = proc->mmupdb) != nil){
 		mmuptefree(proc);
-		mmupdbfree(proc, proc->mmupdb);
+		mmupdbfree(proc, page);
 		proc->mmupdb = nil;
 	}
-	for(page = proc->mmufree; page != nil; page = next){
-		next = page->next;
-		if(--page->ref != 0)
-			panic("mmurelease: page->ref %ld", page->ref);
-		pagechainhead(page);
+	if((page = proc->mmufree) != nil){
+		freepages(page, nil, 0);
+		proc->mmufree = nil;
 	}
-	if(proc->mmufree != nil)
-		pagechaindone();
-	proc->mmufree = nil;
 	if(proc->ldt != nil){
 		free(proc->ldt);
 		proc->ldt = nil;