Re: [PATCH] arm64: break while loop if task had been rescheduled

[tengfeif]

On 2019-05-24 18:41, Mark Rutland wrote:
> On Tue, May 21, 2019 at 05:20:04PM +0800, Tengfei Fan wrote:
> > While printing a task's backtrace and this task isn't
> > current task, it is possible that task's fp and fp+8
> > have the same value, so cannot break the while loop.
> > This can break while loop if this task had been
> > rescheduled during print this task's backtrace.
>
> There are a few cases where backtracing can get stuck in an infinite
> loop. I'd attempted to address that more generally in my
> arm64/robust-stacktrace branch [1].
>
> Looking at tsk->state here is inherently racy, and doesn't solve the
> general case, so I'd prefer to avoid that.
>
> Do my patches help you here? If so, I'm happy to rebase those to
> v5.2-rc1 and repost.

I think your arm64/robust-stacktrace branch [1] can cover my issue, 
please
rebase and reposet

Thanks,
Tengfei Fan

> Thanks,
> Mark.
>
> [1]
> https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/robust-stacktrace
>
> > Signed-off-by: Tengfei Fan <tengfeif@xxxxxxxxxxxxxx>
> > ---
> >  arch/arm64/kernel/traps.c | 23 +++++++++++++++++++++++
> >  1 file changed, 23 insertions(+)
> >
> > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> > index 2975598..9df6e02 100644
> > --- a/arch/arm64/kernel/traps.c
> > +++ b/arch/arm64/kernel/traps.c
> > @@ -103,6 +103,9 @@ void dump_backtrace(struct pt_regs *regs, struct 
> > task_struct *tsk)
> >  {
> >  	struct stackframe frame;
> >  	int skip = 0;
> > +	long cur_state = 0;
> > +	unsigned long cur_sp = 0;
> > +	unsigned long cur_fp = 0;
> >
> >  	pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk);
> >
> > @@ -127,6 +130,9 @@ void dump_backtrace(struct pt_regs *regs, struct 
> > task_struct *tsk)
> >  		 */
> >  		frame.fp = thread_saved_fp(tsk);
> >  		frame.pc = thread_saved_pc(tsk);
> > +		cur_state = tsk->state;
> > +		cur_sp = thread_saved_sp(tsk);
> > +		cur_fp = frame.fp;
> >  	}
> >  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> >  	frame.graph = 0;
> > @@ -134,6 +140,23 @@ void dump_backtrace(struct pt_regs *regs, struct 
> > task_struct *tsk)
> >
> >  	printk("Call trace:\n");
> >  	do {
> > +		if (tsk != current && (cur_state != tsk->state
> > +			/*
> > +			 * We would not be printing backtrace for the task
> > +			 * that has changed state from uninterruptible to
> > +			 * running before hitting the do-while loop but after
> > +			 * saving the current state. If task is in running
> > +			 * state before saving the state, then we may print
> > +			 * wrong call trace or end up in infinite while loop
> > +			 * if *(fp) and *(fp+8) are same. While the situation
> > +			 * will stop print when that task schedule out.
> > +			 */
> > +			|| cur_sp != thread_saved_sp(tsk)
> > +			|| cur_fp != thread_saved_fp(tsk))) {
> > +			printk("The task:%s had been rescheduled!\n",
> > +				tsk->comm);
> > +			break;
> > +		}
> >  		/* skip until specified stack frame */
> >  		if (!skip) {
> >  			dump_backtrace_entry(frame.pc);
> > --
> > The Qualcomm Innovation Center, Inc. is a member of the Code Aurora 
> > Forum,
> > a Linux Foundation Collaborative Project