Remove buffer reference counting, now that it's safe to destroy b_busy after

waking any waiters.

sys/kern/vfs_bio.c

@@ -1,7 +1,7 @@
-/*	$NetBSD: vfs_bio.c,v 1.290 2020/03/14 18:08:39 ad Exp $	*/
+/*	$NetBSD: vfs_bio.c,v 1.291 2020/04/10 17:18:04 ad Exp $	*/
 
 /*-
- * Copyright (c) 2007, 2008, 2009, 2019 The NetBSD Foundation, Inc.
+ * Copyright (c) 2007, 2008, 2009, 2019, 2020 The NetBSD Foundation, Inc.
  * All rights reserved.
  *
  * This code is derived from software contributed to The NetBSD Foundation
@@ -123,7 +123,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: vfs_bio.c,v 1.290 2020/03/14 18:08:39 ad Exp $");
+__KERNEL_RCSID(0, "$NetBSD: vfs_bio.c,v 1.291 2020/04/10 17:18:04 ad Exp $");
 
 #ifdef _KERNEL_OPT
 #include "opt_bufcache.h"
@@ -228,8 +228,6 @@ static int checkfreelist(buf_t *, struct bqueue *, int);
 #endif
 static void biointr(void *);
 static void biodone2(buf_t *);
-static void bref(buf_t *);
-static void brele(buf_t *);
 static void sysctl_kern_buf_setup(void);
 static void sysctl_vm_buf_setup(void);
 
@@ -410,38 +408,6 @@ bremfree(buf_t *bp)
 #endif /* defined(DIAGNOSTIC) */
 }
 
-/*
- * Add a reference to an buffer structure that came from buf_cache.
- */
-static inline void
-bref(buf_t *bp)
-{
-
-	KASSERT(mutex_owned(&bufcache_lock));
-	KASSERT(bp->b_refcnt > 0);
-
-	bp->b_refcnt++;
-}
-
-/*
- * Free an unused buffer structure that came from buf_cache.
- */
-static inline void
-brele(buf_t *bp)
-{
-
-	KASSERT(mutex_owned(&bufcache_lock));
-	KASSERT(bp->b_refcnt > 0);
-
-	if (bp->b_refcnt-- == 1) {
-		buf_destroy(bp);
-#ifdef DEBUG
-		memset((char *)bp, 0, sizeof(*bp));
-#endif
-		pool_cache_put(buf_cache, bp);
-	}
-}
-
 /*
  * note that for some ports this is used by pmap bootstrap code to
  * determine kva size.
@@ -1054,7 +1020,6 @@ brelsel(buf_t *bp, int set)
 	KASSERT(bp != NULL);
 	KASSERT(mutex_owned(&bufcache_lock));
 	KASSERT(!cv_has_waiters(&bp->b_done));
-	KASSERT(bp->b_refcnt > 0);
 
 	SET(bp->b_cflags, set);
 
@@ -1175,14 +1140,19 @@ already_queued:
 	 * prevent a thundering herd: many LWPs simultaneously awakening and
 	 * competing for the buffer's lock.  Testing in 2019 revealed this
 	 * to reduce contention on bufcache_lock tenfold during a kernel
-	 * compile.  Elsewhere, when the buffer is changing identity, being
+	 * compile.  Here and elsewhere, when the buffer is changing
-	 * disposed of, or moving from one list to another, we wake all lock
+	 * identity, being disposed of, or moving from one list to another,
-	 * requestors.
+	 * we wake all lock requestors.
 	 */
+	if (bp->b_bufsize <= 0) {
+		cv_broadcast(&bp->b_busy);
+		buf_destroy(bp);
+#ifdef DEBUG
+		memset((char *)bp, 0, sizeof(*bp));
+#endif
+		pool_cache_put(buf_cache, bp);
+	} else
 		cv_signal(&bp->b_busy);
-
-	if (bp->b_bufsize <= 0)
-		brele(bp);
 }
 
 void
@@ -1494,7 +1464,6 @@ getnewbuf(int slpflag, int slptimeo, int from_bufq)
 	}
 
 	KASSERT(ISSET(bp->b_cflags, BC_BUSY));
-	KASSERT(bp->b_refcnt > 0);
     	KASSERT(!cv_has_waiters(&bp->b_done));
 
 	/*
@@ -1603,7 +1572,6 @@ biowait(buf_t *bp)
 	BIOHIST_FUNC(__func__);
 
 	KASSERT(ISSET(bp->b_cflags, BC_BUSY));
-	KASSERT(bp->b_refcnt > 0);
 
 	SDT_PROBE1(io, kernel, , wait__start, bp);
 
@@ -2172,7 +2140,6 @@ buf_init(buf_t *bp)
 	bp->b_oflags = 0;
 	bp->b_objlock = &buffer_lock;
 	bp->b_iodone = NULL;
-	bp->b_refcnt = 1;
 	bp->b_dev = NODEV;
 	bp->b_vnbufs.le_next = NOLIST;
 	BIO_SETPRIO(bp, BPRIO_DEFAULT);
@@ -2201,7 +2168,6 @@ bbusy(buf_t *bp, bool intr, int timo, kmutex_t *interlock)
 			goto out;
 		}
 		bp->b_cflags |= BC_WANTED;
-		bref(bp);
 		if (interlock != NULL)
 			mutex_exit(interlock);
 		if (intr) {
@@ -2211,16 +2177,18 @@ bbusy(buf_t *bp, bool intr, int timo, kmutex_t *interlock)
 			error = cv_timedwait(&bp->b_busy, &bufcache_lock,
 			    timo);
 		}
-		brele(bp);
+		/*
+		 * At this point the buffer may be gone: don't touch it
+		 * again.  The caller needs to find it again and retry.
+		 */
 		if (interlock != NULL)
 			mutex_enter(interlock);
-		if (error != 0)
+		if (error == 0)
-			goto out;
 			error = EPASSTHROUGH;
-		goto out;
+	} else {
-	}
 		bp->b_cflags |= BC_BUSY;
 		error = 0;
+	}
 
 out:	SDT_PROBE5(io, kernel, , bbusy__done,
 	    bp, intr, timo, interlock, error);

sys/sys/buf.h

@@ -1,4 +1,4 @@
-/*     $NetBSD: buf.h,v 1.131 2019/08/26 10:24:39 msaitoh Exp $ */
+/*     $NetBSD: buf.h,v 1.132 2020/04/10 17:18:04 ad Exp $ */
 
 /*-
  * Copyright (c) 1999, 2000, 2007, 2008 The NetBSD Foundation, Inc.
@@ -149,7 +149,6 @@ struct buf {
 	off_t	b_dcookie;		/* NFS: Offset cookie if dir block */
 
 	kcondvar_t		b_busy;		/* c: threads waiting on buf */
-	u_int			b_refcnt;	/* c: refcount for b_busy */
 	void			*b_unused;	/*  : unused */
 	LIST_ENTRY(buf)		b_hash;		/* c: hash chain */
 	LIST_ENTRY(buf)		b_vnbufs;	/* c: associated vnode */