[PATCH 4.9 028/109] sysrq : fix Show Regs call trace on ARM

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jibin Xu <jibin.xu@windriver.com>


[ Upstream commit b00bebbc301c8e1f74f230dc82282e56b7e7a6db ]

When kernel configuration SMP,PREEMPT and DEBUG_PREEMPT are enabled,
echo 1 >/proc/sys/kernel/sysrq
echo p >/proc/sysrq-trigger
kernel will print call trace as below:

sysrq: SysRq : Show Regs
BUG: using __this_cpu_read() in preemptible [00000000] code: sh/435
caller is __this_cpu_preempt_check+0x18/0x20
Call trace:
[<ffffff8008088e80>] dump_backtrace+0x0/0x1d0
[<ffffff8008089074>] show_stack+0x24/0x30
[<ffffff8008447970>] dump_stack+0x90/0xb0
[<ffffff8008463950>] check_preemption_disabled+0x100/0x108
[<ffffff8008463998>] __this_cpu_preempt_check+0x18/0x20
[<ffffff80084c9194>] sysrq_handle_showregs+0x1c/0x40
[<ffffff80084c9c7c>] __handle_sysrq+0x12c/0x1a0
[<ffffff80084ca140>] write_sysrq_trigger+0x60/0x70
[<ffffff8008251e00>] proc_reg_write+0x90/0xd0
[<ffffff80081f1788>] __vfs_write+0x48/0x90
[<ffffff80081f241c>] vfs_write+0xa4/0x190
[<ffffff80081f3354>] SyS_write+0x54/0xb0
[<ffffff80080833f0>] el0_svc_naked+0x24/0x28

This can be seen on a common board like an r-pi3.
This happens because when echo p >/proc/sysrq-trigger,
get_irq_regs() is called outside of IRQ context,
if preemption is enabled in this situation,kernel will
print the call trace. Since many prior discussions on
the mailing lists have made it clear that get_irq_regs
either just returns NULL or stale data when used outside
of IRQ context,we simply avoid calling it outside of
IRQ context.

Signed-off-by: Jibin Xu <jibin.xu@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/tty/sysrq.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -243,8 +243,10 @@ static void sysrq_handle_showallcpus(int
 	 * architecture has no support for it:
 	 */
 	if (!trigger_all_cpu_backtrace()) {
-		struct pt_regs *regs = get_irq_regs();
+		struct pt_regs *regs = NULL;
 
+		if (in_irq())
+			regs = get_irq_regs();
 		if (regs) {
 			pr_info("CPU%d:\n", smp_processor_id());
 			show_regs(regs);
@@ -263,7 +265,10 @@ static struct sysrq_key_op sysrq_showall
 
 static void sysrq_handle_showregs(int key)
 {
-	struct pt_regs *regs = get_irq_regs();
+	struct pt_regs *regs = NULL;
+
+	if (in_irq())
+		regs = get_irq_regs();
 	if (regs)
 		show_regs(regs);
 	perf_event_print_debug();