[src/trunk]: src/sys/kern Carefully put kernel_lock back the way it was, and ...

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details:   https://anonhg.NetBSD.org/src/rev/0723be2d99e7
branches:  trunk
changeset: 744119:0723be2d99e7
user:      ad <ad%NetBSD.org@localhost>
date:      Fri Jan 24 20:05:15 2020 +0000

description:
Carefully put kernel_lock back the way it was, and add a comment hinting
that changing it is not a good idea, and hopefully nobody will ever try to
change it ever again.

diffstat:

 sys/kern/kern_lock.c  |  30 +++++++++++-------------------
 sys/kern/kern_synch.c |   7 ++++---
 2 files changed, 15 insertions(+), 22 deletions(-)

diffs (119 lines):

diff -r b07d7de82e2b -r 0723be2d99e7 sys/kern/kern_lock.c
--- a/sys/kern/kern_lock.c      Fri Jan 24 18:58:46 2020 +0000
+++ b/sys/kern/kern_lock.c      Fri Jan 24 20:05:15 2020 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: kern_lock.c,v 1.166 2020/01/22 13:19:33 ad Exp $       */
+/*     $NetBSD: kern_lock.c,v 1.167 2020/01/24 20:05:15 ad Exp $       */
 
 /*-
  * Copyright (c) 2002, 2006, 2007, 2008, 2009, 2020 The NetBSD Foundation, Inc.
@@ -31,7 +31,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_lock.c,v 1.166 2020/01/22 13:19:33 ad Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_lock.c,v 1.167 2020/01/24 20:05:15 ad Exp $");
 
 #include <sys/param.h>
 #include <sys/proc.h>
@@ -156,6 +156,11 @@
 
 /*
  * Acquire 'nlocks' holds on the kernel lock.
+ *
+ * Although it may not look it, this is one of the most central, intricate
+ * routines in the kernel, and tons of code elsewhere depends on its exact
+ * behaviour.  If you change something in here, expect it to bite you in the
+ * rear.
  */
 void
 _kernel_lock(int nlocks)
@@ -164,7 +169,6 @@
        LOCKSTAT_TIMER(spintime);
        LOCKSTAT_FLAG(lsflag);
        struct lwp *owant;
-       u_int count;
 #ifdef LOCKDEBUG
        u_int spins = 0;
 #endif
@@ -210,16 +214,15 @@
         */
        membar_producer();
        owant = ci->ci_biglock_wanted;
+       ci->ci_biglock_wanted = l;
 
        /*
-        * Stay pinned to the CPU and spin until we acquire the lock.  Once
-        * we have it, record the time spent with lockstat.
+        * Spin until we acquire the lock.  Once we have it, record the
+        * time spent with lockstat.
         */
-       l->l_nopreempt++;
        LOCKSTAT_ENTER(lsflag);
        LOCKSTAT_START_TIMER(lsflag, spintime);
 
-       count = SPINLOCK_BACKOFF_MIN;
        do {
                splx(s);
                while (__SIMPLELOCK_LOCKED_P(kernel_lock)) {
@@ -230,24 +233,12 @@
                                        _KERNEL_LOCK_ABORT("spinout");
                        }
 #endif
-                       SPINLOCK_BACKOFF(count);
-                       if (count == SPINLOCK_BACKOFF_MAX) {
-                               /* Ok, waiting for real. */
-                               ci->ci_biglock_wanted = l;
-                       }
                }
                s = splvm();
        } while (!__cpu_simple_lock_try(kernel_lock));
 
-       /*
-        * Got it; not re-enable preemption, although we now can't do a
-        * preemption as kernel_lock is held!
-        */
-       l->l_nopreempt--;
        ci->ci_biglock_count = nlocks;
        l->l_blcnt = nlocks;
-       splx(s);
-
        LOCKSTAT_STOP_TIMER(lsflag, spintime);
        LOCKDEBUG_LOCKED(kernel_lock_dodebug, kernel_lock, NULL,
            RETURN_ADDRESS, 0);
@@ -256,6 +247,7 @@
                    LB_KERNEL_LOCK | LB_SPIN, 1, spintime, RETURN_ADDRESS);
        }
        LOCKSTAT_EXIT(lsflag);
+       splx(s);
 
        /*
         * Now that we have kernel_lock, reset ci_biglock_wanted.  This
diff -r b07d7de82e2b -r 0723be2d99e7 sys/kern/kern_synch.c
--- a/sys/kern/kern_synch.c     Fri Jan 24 18:58:46 2020 +0000
+++ b/sys/kern/kern_synch.c     Fri Jan 24 20:05:15 2020 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: kern_synch.c,v 1.337 2020/01/22 13:19:33 ad Exp $      */
+/*     $NetBSD: kern_synch.c,v 1.338 2020/01/24 20:05:15 ad Exp $      */
 
 /*-
  * Copyright (c) 1999, 2000, 2004, 2006, 2007, 2008, 2009, 2019
@@ -69,7 +69,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_synch.c,v 1.337 2020/01/22 13:19:33 ad Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_synch.c,v 1.338 2020/01/24 20:05:15 ad Exp $");
 
 #include "opt_kstack.h"
 #include "opt_dtrace.h"
@@ -353,7 +353,8 @@
                        break;
                }
                s = splsched();
-               if (__predict_false(l->l_blcnt != 0)) {
+               if (__predict_false(l->l_blcnt != 0 ||
+                   curcpu()->ci_biglock_wanted != NULL)) {
                        /* Hold or want kernel_lock, code is not MT safe. */
                        splx(s);
                        if ((dop & DOPREEMPT_COUNTED) == 0) {