The patch below does not apply to the 4.20-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From f731a8e89f8c78985707c626680f3e24c7a60772 Mon Sep 17 00:00:00 2001

From: Vineet Gupta vgupta@synopsys.com Date: Tue, 18 Dec 2018 10:39:58 -0800 Subject: [PATCH] ARC: show_regs: lockdep: re-enable preemption

signal handling core calls show_regs() with preemption disabled which on ARC takes mmap_sem for mm/vma access, causing lockdep splat.

| [ARCLinux]# ./segv-null-ptr | potentially unexpected fatal signal 11. | BUG: sleeping function called from invalid context at kernel/fork.c:1011 | in_atomic(): 1, irqs_disabled(): 0, pid: 70, name: segv-null-ptr | no locks held by segv-null-ptr/70. | CPU: 0 PID: 70 Comm: segv-null-ptr Not tainted 4.18.0+ #69 | | Stack Trace: | arc_unwind_core+0xcc/0x100 | ___might_sleep+0x17a/0x190 | mmput+0x16/0xb8 | show_regs+0x52/0x310 | get_signal+0x5ee/0x610 | do_signal+0x2c/0x218 | resume_user_mode_begin+0x90/0xd8

Workaround by re-enabling preemption temporarily.

Note that the preemption disabling in core code around show_regs() was introduced by commit 3a9f84d354ce ("signals, debug: fix BUG: using smp_processor_id() in preemptible code in print_fatal_signal()")

to silence a differnt lockdep seen on x86 bakc in 2009.

Cc: stable@vger.kernel.org Signed-off-by: Vineet Gupta vgupta@synopsys.com

diff --git a/arch/arc/kernel/troubleshoot.c b/arch/arc/kernel/troubleshoot.c index 5c6663321e87..215f515442e0 100644 --- a/arch/arc/kernel/troubleshoot.c +++ b/arch/arc/kernel/troubleshoot.c @@ -179,6 +179,12 @@ void show_regs(struct pt_regs *regs) struct task_struct *tsk = current; struct callee_regs *cregs;

+ /* + * generic code calls us with preemption disabled, but some calls + * here could sleep, so re-enable to avoid lockdep splat + */ + preempt_enable(); + print_task_path_n_nm(tsk); show_regs_print_info(KERN_INFO);

@@ -221,6 +227,8 @@ void show_regs(struct pt_regs *regs) cregs = (struct callee_regs *)current->thread.callee_reg; if (cregs) show_callee_regs(cregs); + + preempt_disable(); }

void show_kernel_fault_diag(const char *str, struct pt_regs *regs,