Date: Thu, 3 Nov 2005 13:27:13 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-current@freebsd.org, lonnie.vanzandt@ngc.com Cc: marcel@freebsd.org Subject: Re: Cdiff patch for kernel gdb and mi_switch panic in freebsd 5.4 STABLE Message-ID: <200511031327.18011.jhb@freebsd.org> In-Reply-To: <200509220742.10364.lonnie.vanzandt@ngc.com> References: <200509220742.10364.lonnie.vanzandt@ngc.com>
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On Sunday 09 October 2005 05:49 pm, Lonnie VanZandt wrote:
> Attached is the patch for the revised subr_kdb.c from FreeBSD 5.4 STABLE.
> (the rcsid is __FBSDID("$FreeBSD: src/sys/kern/subr_kdb.c,v 1.5.2.2.2.1
> 2005/05/01 05:38:14 dwhite Exp $"); )
I've looked at this, but I think t could maybe be done slightly differently.
Here's a suggested patch that would close the race you are seeing I think
while allowing semantics such that if two CPUs try to enter KDB at the same
time, they would serialize and the second CPU would enter kdb after the first
had exited. Could you at least test it to see if it addresses your race
condition?
--- //depot/projects/smpng/sys/kern/subr_kdb.c 2005/10/27 19:51:50
+++ //depot/user/jhb/ktrace/kern/subr_kdb.c 2005/11/03 18:24:38
@@ -39,6 +39,7 @@
#include <sys/smp.h>
#include <sys/sysctl.h>
+#include <machine/cpu.h>
#include <machine/kdb.h>
#include <machine/pcb.h>
@@ -462,12 +463,21 @@
return (0);
/* We reenter the debugger through kdb_reenter(). */
- if (kdb_active)
+ if (kdb_active == PCPU_GET(cpuid) + 1)
return (0);
critical_enter();
- kdb_active++;
+ /*
+ * If more than one CPU tries to enter KDB at the same time
+ * then force them to serialize and go one at a time.
+ */
+ while (!atomic_cmpset_int(&kdb_active, 0, PCPU_GET(cpuid) + 1)) {
+ critical_exit();
+ while (kdb_active)
+ cpu_spinwait();
+ critical_enter();
+ }
#ifdef SMP
if ((did_stop_cpus = kdb_stop_cpus) != 0)
@@ -484,13 +494,17 @@
handled = kdb_dbbe->dbbe_trap(type, code);
+ /*
+ * We have to exit KDB before resuming the other CPUs so that they
+ * may run in a debugger-less context.
+ */
+ kdb_active = 0;
+
#ifdef SMP
if (did_stop_cpus)
restart_cpus(stopped_cpus);
#endif
- kdb_active--;
-
critical_exit();
return (handled);
--
John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve" = http://www.FreeBSD.org
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