[PATCH] kdump: sysrq trigger mechanism for kexec based crashdumps

Add a sysrq-trigger mechanism for kexec based crashdumps. Alt-Sysrq-c
triggers a kexec based crashdump.

Signed-off-by: Hariprasad Nellitheertha <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/sysrq.txt

···
 'b'     - Will immediately reboot the system without syncing or unmounting
           your disks.
 
+'c'	- Will perform a kexec reboot in order to take a crashdump.
+
 'o'     - Will shut your system off (if configured and supported).
 
 's'     - Will attempt to sync all mounted filesystems.
···
 
 re'B'oot is good when you're unable to shut down. But you should also 'S'ync
 and 'U'mount first.
+
+'C'rashdump can be used to manually trigger a crashdump when the system is hung.
+The kernel needs to have been built with CONFIG_KEXEC enabled.
 
 'S'ync is great when your system is locked up, it allows you to sync your
 disks and will certainly lessen the chance of data loss and fscking. Note

drivers/char/sysrq.c

···
 #include <linux/spinlock.h>
 #include <linux/vt_kern.h>
 #include <linux/workqueue.h>
+#include <linux/kexec.h>
 
 #include <asm/ptrace.h>
 
···
 	.enable_mask	= SYSRQ_ENABLE_KEYBOARD,
 };
 #endif /* CONFIG_VT */
+
+#ifdef CONFIG_KEXEC
+/* crashdump sysrq handler */
+static void sysrq_handle_crashdump(int key, struct pt_regs *pt_regs,
+				struct tty_struct *tty)
+{
+	crash_kexec();
+}
+static struct sysrq_key_op sysrq_crashdump_op = {
+	.handler	= sysrq_handle_crashdump,
+	.help_msg	= "Crashdump",
+	.action_msg	= "Trigger a crashdump",
+	.enable_mask	= SYSRQ_ENABLE_DUMP,
+};
+#endif
 
 /* reboot sysrq handler */
 static void sysrq_handle_reboot(int key, struct pt_regs *pt_regs,
···
 		 it is handled specially on the sparc
 		 and will never arrive */
 /* b */	&sysrq_reboot_op,
-/* c */ NULL,
-/* d */	NULL,
+#ifdef CONFIG_KEXEC
+/* c */ &sysrq_crashdump_op,
+#else
+/* c */	NULL,
+#endif
+/* d */ NULL,
 /* e */	&sysrq_term_op,
 /* f */	&sysrq_moom_op,
 /* g */	NULL,