Commit b00bebbc authored Sep 10, 2017 by Jibin Xu Committed by Greg Kroah-Hartman Oct 04, 2017

sysrq : fix Show Regs call trace on ARM



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>

parent 4038e348

drivers/tty/sysrq.c

+7 −2

Original line number	Diff line number	Diff line
		@@ -245,8 +245,10 @@ static void sysrq_handle_showallcpus(int key)
		* 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);
		@@ -265,7 +267,10 @@ static struct sysrq_key_op sysrq_showallcpus_op = {

		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();

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