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SubjectRe: [PATCH] X86: reboot-notify additions

On Thu, Jun 19, 2008 at 01:02:14PM +0200, Ingo Molnar wrote:
>
> * Cliff Wickman <cpw@sgi.com> wrote:
>
> > From: Cliff Wickman <cpw@sgi.com>
> >
> > X86 reboot-notify additions.

As Andi Kleen pointed out, this is not X86-specific. (it started out
to be, but what I hoped to achieve for X86 turns out to be generic).

> > This patch adds scans of the "reboot_notifier_list" callback chain in
> > a three other places where the kernel is being stopped and/or restarted.
> >
> > Adds calls to blocking_notifier_call_chain() in:
> > crash_kexec(), emergency_restart(), sys_kexec_load()
> >
> > In the crash_kexec() and emergency_restart() cases it is indicated to the
> > called-back function that the system is not in a sane state, so that
> > it can avoid taking a lock or some such potentially blocking action.
> >
> > These callbacks are important to a partition system. The stopped kernel needs
> > to inform other partitions of their need to disconnect (stop sharing memory).
> >
> > Diffed against 2.6.26-rc6
> >
> > Signed-off-by: Cliff Wickman <cpw@sgi.com>
> > ---
> > include/linux/notifier.h | 4 ++++
> > kernel/kexec.c | 5 +++++
> > kernel/sys.c | 1 +
> > 3 files changed, 10 insertions(+)
> >
> > Index: linux/include/linux/notifier.h
> > ===================================================================
> > --- linux.orig/include/linux/notifier.h
> > +++ linux/include/linux/notifier.h
> > @@ -202,6 +202,10 @@ static inline int notifier_to_errno(int
> > #define SYS_RESTART SYS_DOWN
> > #define SYS_HALT 0x0002 /* Notify of system halt */
> > #define SYS_POWER_OFF 0x0003 /* Notify of system power off */
> > +#define SYS_INSANE 0x0004 /* Notify of system error/panic/oops */
> > +/* For the SYS_INSANE case, no locks should be taken by the called-back
> > + * function. The kernel is ready for an immediate reboot.
> > + */
> >
> > #define NETLINK_URELEASE 0x0001 /* Unicast netlink socket released */
> >
> > Index: linux/kernel/kexec.c
> > ===================================================================
> > --- linux.orig/kernel/kexec.c
> > +++ linux/kernel/kexec.c
> > @@ -1001,6 +1001,9 @@ asmlinkage long sys_kexec_load(unsigned
> > if (result)
> > goto out;
> > }
> > +
> > + blocking_notifier_call_chain(&reboot_notifier_list, SYS_RESTART, NULL);
> > +
> > /* Install the new kernel, and Uninstall the old */
> > image = xchg(dest_image, image);
> >
> > @@ -1068,6 +1071,8 @@ void crash_kexec(struct pt_regs *regs)
> > if (!locked) {
> > if (kexec_crash_image) {
> > struct pt_regs fixed_regs;
> > + blocking_notifier_call_chain(&reboot_notifier_list,
> > + SYS_INSANE, NULL);
> > crash_setup_regs(&fixed_regs, regs);
> > crash_save_vmcoreinfo();
> > machine_crash_shutdown(&fixed_regs);
> > Index: linux/kernel/sys.c
> > ===================================================================
> > --- linux.orig/kernel/sys.c
> > +++ linux/kernel/sys.c
> > @@ -270,6 +270,7 @@ out_unlock:
> > */
> > void emergency_restart(void)
> > {
> > + blocking_notifier_call_chain(&reboot_notifier_list, SYS_INSANE, NULL);
> > machine_emergency_restart();
> > }
> > EXPORT_SYMBOL_GPL(emergency_restart);
>
> i dont think this is a good idea. reboot_notifier_list is a blocking
> notifier, i.e. it comes with a notifier->rwsem read-write mutex that is
> taken when blocking_notifier_call_chain() is executed.
>
> i.e. this patch puts a sleeping mutex operation (a down_read()) into a
> highly critical code path of the kernel. This will decrease the
> reliability of the kernel.

Andi pointed this out, too.

For these emergency cases (I'll change "SYS_INSANE" to "SYS_EMERGENCY")
I probably should be using raw_notifier_call_chain(), which requires
a slightly different form of list header but doesn't try to protect
against someone else adding to the notifier list.
>
> what exactly are you trying to achieve?
>
> Ingo

The impetus for these additions is to call back a driver in every case
that the kernel is going down. In a partitioned system we need such a
driver to inform all other partitions that they need to disconnect from
the rebooting/halting/panicing partition (kernel image). If they are not
informed, they may bring themselves crashing down as well.
(xpc is such a cross_partition driver)



I propose this revision of the patch instead:


Subject: [PATCH] reboot-notify additions

reboot-notify additions

This patch adds scans of the "reboot_notifier_list" callback chain in
the remaining places where the kernel is being stopped and/or restarted.

Adds 3 calls to {raw|blocking}_notifier_call_chain() in:
crash_kexec(), sys_kexec_load(), emergency_restart()

Diffed against 2.6.26-rc6

Signed-off-by: Cliff Wickman <cpw@sgi.com>
---
include/linux/notifier.h | 3 +++
kernel/kexec.c | 10 ++++++++++
kernel/sys.c | 7 +++++++
3 files changed, 20 insertions(+)

Index: linux/include/linux/notifier.h
===================================================================
--- linux.orig/include/linux/notifier.h
+++ linux/include/linux/notifier.h
@@ -202,6 +202,9 @@ static inline int notifier_to_errno(int
#define SYS_RESTART SYS_DOWN
#define SYS_HALT 0x0002 /* Notify of system halt */
#define SYS_POWER_OFF 0x0003 /* Notify of system power off */
+#define SYS_EMERGENCY 0x0004 /* Notify of system error/panic/oops */
+/* For the SYS_EMERGENCY case, no locks should be taken by the called-back
+ * function. */

#define NETLINK_URELEASE 0x0001 /* Unicast netlink socket released */

Index: linux/kernel/kexec.c
===================================================================
--- linux.orig/kernel/kexec.c
+++ linux/kernel/kexec.c
@@ -24,6 +24,7 @@
#include <linux/utsrelease.h>
#include <linux/utsname.h>
#include <linux/numa.h>
+#include <linux/notifier.h>

#include <asm/page.h>
#include <asm/uaccess.h>
@@ -1001,6 +1002,9 @@ asmlinkage long sys_kexec_load(unsigned
if (result)
goto out;
}
+
+ blocking_notifier_call_chain(&reboot_notifier_list, SYS_RESTART, NULL);
+
/* Install the new kernel, and Uninstall the old */
image = xchg(dest_image, image);

@@ -1063,11 +1067,17 @@ void crash_kexec(struct pt_regs *regs)
* If the crash kernel was not located in a fixed area
* of memory the xchg(&kexec_crash_image) would be
* sufficient. But since I reuse the memory...
+ *
+ * The reboot_notifier_list uses a header for a blocking-form scan.
+ * Use a local header suitable for a non-blocking scan.
*/
locked = xchg(&kexec_lock, 1);
if (!locked) {
if (kexec_crash_image) {
struct pt_regs fixed_regs;
+ struct raw_notifier_head rh;
+ rh.head = reboot_notifier_list.head;
+ raw_notifier_call_chain(&rh, SYS_EMERGENCY, NULL);
crash_setup_regs(&fixed_regs, regs);
crash_save_vmcoreinfo();
machine_crash_shutdown(&fixed_regs);
Index: linux/kernel/sys.c
===================================================================
--- linux.orig/kernel/sys.c
+++ linux/kernel/sys.c
@@ -267,9 +267,16 @@ out_unlock:
* reboot the system. This is called when we know we are in
* trouble so this is our best effort to reboot. This is
* safe to call in interrupt context.
+ *
+ * The reboot_notifier_list uses a header for a blocking-form scan.
+ * Use a local header suitable for a non-blocking scan.
*/
void emergency_restart(void)
{
+ struct raw_notifier_head rh;
+
+ rh.head = reboot_notifier_list.head;
+ raw_notifier_call_chain(&rh, SYS_EMERGENCY, NULL);
machine_emergency_restart();
}
EXPORT_SYMBOL_GPL(emergency_restart);
--
Cliff Wickman
Silicon Graphics, Inc.
cpw@sgi.com
(651) 683-3824


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