[PATCH] arm64: avoid race condition issue in dump_backtrace

[ji.zhang@mediatek.com (Ji.Zhang)]

Dear Baruch,

Thank you for your kind and careful examination.
That is my stupid fault due to I use different environments to do
verification and submit patch which I do the merge manually.
Anyway, thanks so much for the reminder, I will send another patch.

BRs,
Ji 

On Thu, 2018-03-22 at 06:57 +0200, Baruch Siach wrote:
> Hi Ji Zhang,
> 
> On Thu, Mar 22, 2018 at 11:06:00AM +0800, Ji Zhang wrote:
> > When we dump the backtrace of some specific task, there is a potential race
> > condition due to the task may be running on other cores if SMP enabled.
> > That is because for current implementation, if the task is not the current
> > task, we will get the registers used for unwind from cpu_context saved in
> > thread_info, which is the snapshot before context switch, but if the task
> > is running on other cores, the registers and the content of stack are
> > changed.
> > This may cause that we get the wrong backtrace or incomplete backtrace or
> > even crash the kernel.
> > To avoid this case, do not dump the backtrace of the tasks which are
> > running on other cores.
> > This patch cannot solve the issue completely but can shrink the window of
> > race condition.
> > 
> > Signed-off-by: Ji Zhang <ji.zhang@mediatek.com>
> > ---
> >  arch/arm64/kernel/traps.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> > index eb2d151..95749364 100644
> > --- a/arch/arm64/kernel/traps.c
> > +++ b/arch/arm64/kernel/traps.c
> > @@ -113,6 +113,9 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
> >  	if (tsk == current) {
> >  		frame.fp = (unsigned long)__builtin_frame_address(0);
> >  		frame.pc = (unsigned long)dump_backtrace;
> > +	else if (tsk->state == TASK_RUNNING) {
> 
> Missing closing brace. Does this build?
> 
> > +		pr_notice("Do not dump other running tasks\n");
> > +		return;
> >  	} else {
> >  		/*
> >  		 * task blocked in __switch_to
> 
> baruch
>