This is a note to let you know that I've just added the patch titled

sysrq : fix Show Regs call trace on ARM

to the 4.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: sysrq-fix-show-regs-call-trace-on-arm.patch and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From foo@baz Wed Dec 6 16:43:17 CET 2017

From: Jibin Xu jibin.xu@windriver.com Date: Sun, 10 Sep 2017 20:11:42 -0700 Subject: sysrq : fix Show Regs call trace on ARM

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 @@ -237,8 +237,10 @@ 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); @@ -257,7 +259,10 @@ 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();

Patches currently in stable-queue which might be from jibin.xu@windriver.com are

queue-4.4/sysrq-fix-show-regs-call-trace-on-arm.patch