[src/trunk]: src/sys Remove buffer reference counting, now that it's safe to ...

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details:   https://anonhg.NetBSD.org/src/rev/adf2937afd52
branches:  trunk
changeset: 970998:adf2937afd52
user:      ad <ad%NetBSD.org@localhost>
date:      Fri Apr 10 17:18:04 2020 +0000

description:
Remove buffer reference counting, now that it's safe to destroy b_busy after
waking any waiters.

diffstat:

 sys/kern/vfs_bio.c |  80 ++++++++++++++++-------------------------------------
 sys/sys/buf.h      |   3 +-
 2 files changed, 25 insertions(+), 58 deletions(-)

diffs (180 lines):

diff -r 9ab6124ddb93 -r adf2937afd52 sys/kern/vfs_bio.c
--- a/sys/kern/vfs_bio.c        Fri Apr 10 17:16:21 2020 +0000
+++ b/sys/kern/vfs_bio.c        Fri Apr 10 17:18:04 2020 +0000
@@ -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 @@
 #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);
 
@@ -411,38 +409,6 @@
 }
 
 /*
- * 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 @@
        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 @@
         * 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
-        * disposed of, or moving from one list to another, we wake all lock
-        * requestors.
+        * compile.  Here and elsewhere, when the buffer is changing
+        * identity, being disposed of, or moving from one list to another,
+        * we wake all lock requestors.
         */
-       cv_signal(&bp->b_busy);
-
-       if (bp->b_bufsize <= 0)
-               brele(bp);
+       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);
 }
 
 void
@@ -1494,7 +1464,6 @@
        }
 
        KASSERT(ISSET(bp->b_cflags, BC_BUSY));
-       KASSERT(bp->b_refcnt > 0);
        KASSERT(!cv_has_waiters(&bp->b_done));
 
        /*
@@ -1603,7 +1572,6 @@
        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 @@
        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 @@
                        goto out;
                }
                bp->b_cflags |= BC_WANTED;
-               bref(bp);
                if (interlock != NULL)
                        mutex_exit(interlock);
                if (intr) {
@@ -2211,16 +2177,18 @@
                        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)
-                       goto out;
-               error = EPASSTHROUGH;
-               goto out;
+               if (error == 0)
+                       error = EPASSTHROUGH;
+       } else {
+               bp->b_cflags |= BC_BUSY;
+               error = 0;
        }
-       bp->b_cflags |= BC_BUSY;
-       error = 0;
 
 out:   SDT_PROBE5(io, kernel, , bbusy__done,
            bp, intr, timo, interlock, error);
diff -r 9ab6124ddb93 -r adf2937afd52 sys/sys/buf.h
--- a/sys/sys/buf.h     Fri Apr 10 17:16:21 2020 +0000
+++ b/sys/sys/buf.h     Fri Apr 10 17:18:04 2020 +0000
@@ -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 @@
        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 */