[PATCH] /proc/sysrq-trigger can now pause processing for one second

[Tomas Mudrunka <tomas.mudrunka@gmail.com>]

Writing ',' to /proc/sysrq-trigger now causes processing to
pause for one second.

This is useful, because recently accepted patch allows
to write multiple keys at once to /proc/sysrq-trigger.
But it might be desirable to add slight delay between actions.

Eg. between (e)TERM and (i)KILL it makes sense to put slight delay,
so processes have chance to run TERM handlers before being KILLed.

Now we can send TERM, wait for two seconds and KILL like this:

echo _e,,i > /proc/sysrq-trigger

Originaly i've tested doing this as handler registered in
sysrq_key_table[], but that would cause delay to occur while
holding sysrq rcu lock in __handle_sysrq(), therefore i've decided
to implement this in write_sysrq_trigger() instead to allow
proper use of msleep() instead of mdelay() in locked context.

This means it will only be possible to invoke the delay using
/proc/sysrq-trigger, but there is little point in doing that
interactively using keyboard anyway.

Depends-on: /proc/sysrq-trigger: accept multiple keys at once

Signed-off-by: Tomas Mudrunka <tomas.mudrunka@gmail.com>
---
 Documentation/admin-guide/sysrq.rst |  2 ++
 drivers/tty/sysrq.c                 | 16 ++++++++++++++--
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/sysrq.rst b/Documentation/admin-guide/sysrq.rst
index 2f2e5bd44..b798a2695 100644
--- a/Documentation/admin-guide/sysrq.rst
+++ b/Documentation/admin-guide/sysrq.rst
@@ -161,6 +161,8 @@ Command	    Function
             will be printed to your console. (``0``, for example would make
             it so that only emergency messages like PANICs or OOPSes would
             make it to your console.)
+
+``,``	    Wait for one second (only for use with /proc/sysrq-trigger batch)
 =========== ===================================================================
 
 Okay, so what can I use them for?
diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 02217e3c9..a19ce0865 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -51,6 +51,7 @@
 #include <linux/syscalls.h>
 #include <linux/of.h>
 #include <linux/rcupdate.h>
+#include <linux/delay.h>
 
 #include <asm/ptrace.h>
 #include <asm/irq_regs.h>
@@ -1166,10 +1167,21 @@ static ssize_t write_sysrq_trigger(struct file *file, const char __user *buf,
 		if (get_user(c, buf + i))
 			return -EFAULT;
 
-		if (c == '_')
+		switch (c) {
+
+		case '_':
 			bulk = true;
-		else
+			break;
+
+		case ',':
+			msleep(1000);
+			break;
+
+		default:
 			__handle_sysrq(c, false);
+			break;
+
+		}
 
 		if (!bulk)
 			break;
-- 
2.43.0