[PATCH -mm] extend sysrq-p functionality to cover all CPUs

[Rik van Riel <riel@surriel.com>]

SysRP-P is not all that useful on SMP systems, since the sysrq
irq rarely ends up on the CPU that we actually want to investigate.

This patch extends sysrq-p to print a backtrace for every CPU,
not just the lucky one that gets the sysrq irq.  With this patch,
"echo p > /proc/sysrq-trigger" does something useful.

Signed-off-by: Rik van Riel <riel@redhat.com>

diff -up linux-2.6.25-rc3-mm1/drivers/char/sysrq.c.multicpu linux-2.6.25-rc3-mm1/drivers/char/sysrq.c
--- linux-2.6.25-rc3-mm1/drivers/char/sysrq.c.multicpu	2008-03-09 20:22:17.000000000 -0400
+++ linux-2.6.25-rc3-mm1/drivers/char/sysrq.c	2008-03-09 21:54:02.000000000 -0400
@@ -196,11 +196,29 @@ static struct sysrq_key_op sysrq_showloc
 #define sysrq_showlocks_op (*(struct sysrq_key_op *)0)
 #endif
 
+static DEFINE_SPINLOCK(show_lock);
+static void showacpu(void *dummy)
+{
+	struct pt_regs *regs = get_irq_regs();
+
+	spin_lock(&show_lock);
+	printk("CPU%d:\n", smp_processor_id());
+	show_stack(NULL, NULL);
+	spin_unlock(&show_lock);
+}
+static void sysrq_showregs_othercpus(struct work_struct *dummy)
+{
+	smp_call_function(showacpu, NULL, 0, 0);
+}
+static DECLARE_WORK(sysrq_showregs, sysrq_showregs_othercpus);
 static void sysrq_handle_showregs(int key, struct tty_struct *tty)
 {
 	struct pt_regs *regs = get_irq_regs();
-	if (regs)
+	if (regs) {
+		printk("CPU%d:\n", smp_processor_id());
 		show_regs(regs);
+	}
+	schedule_work(&sysrq_showregs);
 }
 static struct sysrq_key_op sysrq_showregs_op = {
 	.handler	= sysrq_handle_showregs,