[RFC 1/5] tty/sysrq: Make sysrq handler NMI aware

[Sumit Garg <sumit.garg@linaro.org>]

In a future patch we will add support to the serial core to make it
possible to trigger a magic sysrq from an NMI context. Prepare for this
by marking some sysrq actions as NMI safe. Safe actions will be allowed
to run from NMI context whilst that cannot run from an NMI will be queued
as irq_work for later processing.

A particular sysrq handler is only marked as NMI safe in case the handler
isn't contending for any synchronization primitives as in NMI context
they are expected to cause deadlocks. Note that the debug sysrq do not
contend for any synchronization primitives. It does call kgdb_breakpoint()
to provoke a trap but that trap handler should be NMI safe on
architectures that implement an NMI.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
---
 drivers/tty/sysrq.c       | 33 ++++++++++++++++++++++++++++++++-
 include/linux/sysrq.h     |  1 +
 kernel/debug/debug_core.c |  1 +
 3 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 7c95afa9..8017e33 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -50,6 +50,8 @@
 #include <linux/syscalls.h>
 #include <linux/of.h>
 #include <linux/rcupdate.h>
+#include <linux/irq_work.h>
+#include <linux/kfifo.h>
 
 #include <asm/ptrace.h>
 #include <asm/irq_regs.h>
@@ -111,6 +113,7 @@ static const struct sysrq_key_op sysrq_loglevel_op = {
 	.help_msg	= "loglevel(0-9)",
 	.action_msg	= "Changing Loglevel",
 	.enable_mask	= SYSRQ_ENABLE_LOG,
+	.nmi_safe	= true,
 };
 
 #ifdef CONFIG_VT
@@ -157,6 +160,7 @@ static const struct sysrq_key_op sysrq_crash_op = {
 	.help_msg	= "crash(c)",
 	.action_msg	= "Trigger a crash",
 	.enable_mask	= SYSRQ_ENABLE_DUMP,
+	.nmi_safe	= true,
 };
 
 static void sysrq_handle_reboot(int key)
@@ -170,6 +174,7 @@ static const struct sysrq_key_op sysrq_reboot_op = {
 	.help_msg	= "reboot(b)",
 	.action_msg	= "Resetting",
 	.enable_mask	= SYSRQ_ENABLE_BOOT,
+	.nmi_safe	= true,
 };
 
 const struct sysrq_key_op *__sysrq_reboot_op = &sysrq_reboot_op;
@@ -217,6 +222,7 @@ static const struct sysrq_key_op sysrq_showlocks_op = {
 	.handler	= sysrq_handle_showlocks,
 	.help_msg	= "show-all-locks(d)",
 	.action_msg	= "Show Locks Held",
+	.nmi_safe	= true,
 };
 #else
 #define sysrq_showlocks_op (*(const struct sysrq_key_op *)NULL)
@@ -289,6 +295,7 @@ static const struct sysrq_key_op sysrq_showregs_op = {
 	.help_msg	= "show-registers(p)",
 	.action_msg	= "Show Regs",
 	.enable_mask	= SYSRQ_ENABLE_DUMP,
+	.nmi_safe	= true,
 };
 
 static void sysrq_handle_showstate(int key)
@@ -326,6 +333,7 @@ static const struct sysrq_key_op sysrq_ftrace_dump_op = {
 	.help_msg	= "dump-ftrace-buffer(z)",
 	.action_msg	= "Dump ftrace buffer",
 	.enable_mask	= SYSRQ_ENABLE_DUMP,
+	.nmi_safe	= true,
 };
 #else
 #define sysrq_ftrace_dump_op (*(const struct sysrq_key_op *)NULL)
@@ -538,6 +546,23 @@ static void __sysrq_put_key_op(int key, const struct sysrq_key_op *op_p)
                 sysrq_key_table[i] = op_p;
 }
 
+#define SYSRQ_NMI_FIFO_SIZE	64
+static DEFINE_KFIFO(sysrq_nmi_fifo, int, SYSRQ_NMI_FIFO_SIZE);
+
+static void sysrq_do_nmi_work(struct irq_work *work)
+{
+	const struct sysrq_key_op *op_p;
+	int key;
+
+	while (kfifo_out(&sysrq_nmi_fifo, &key, 1)) {
+		op_p = __sysrq_get_key_op(key);
+		if (op_p)
+			op_p->handler(key);
+	}
+}
+
+static DEFINE_IRQ_WORK(sysrq_nmi_work, sysrq_do_nmi_work);
+
 void __handle_sysrq(int key, bool check_mask)
 {
 	const struct sysrq_key_op *op_p;
@@ -568,7 +593,13 @@ void __handle_sysrq(int key, bool check_mask)
 		if (!check_mask || sysrq_on_mask(op_p->enable_mask)) {
 			pr_info("%s\n", op_p->action_msg);
 			console_loglevel = orig_log_level;
-			op_p->handler(key);
+
+			if (in_nmi() && !op_p->nmi_safe) {
+				kfifo_in(&sysrq_nmi_fifo, &key, 1);
+				irq_work_queue(&sysrq_nmi_work);
+			} else {
+				op_p->handler(key);
+			}
 		} else {
 			pr_info("This sysrq operation is disabled.\n");
 			console_loglevel = orig_log_level;
diff --git a/include/linux/sysrq.h b/include/linux/sysrq.h
index 3a582ec..630b5b9 100644
--- a/include/linux/sysrq.h
+++ b/include/linux/sysrq.h
@@ -34,6 +34,7 @@ struct sysrq_key_op {
 	const char * const help_msg;
 	const char * const action_msg;
 	const int enable_mask;
+	const bool nmi_safe;
 };
 
 #ifdef CONFIG_MAGIC_SYSRQ
diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
index 9e59347..2b51173 100644
--- a/kernel/debug/debug_core.c
+++ b/kernel/debug/debug_core.c
@@ -943,6 +943,7 @@ static const struct sysrq_key_op sysrq_dbg_op = {
 	.handler	= sysrq_handle_dbg,
 	.help_msg	= "debug(g)",
 	.action_msg	= "DEBUG",
+	.nmi_safe	= true,
 };
 #endif
 
-- 
2.7.4