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Date:   Thu, 30 May 2019 10:41:11 +0800
From:   tengfeif@...eaurora.org
To:     Mark Rutland <mark.rutland@....com>
Cc:     catalin.marinas@....com, will.deacon@....com, marc.zyngier@....com,
        anshuman.khandual@....com, andreyknvl@...gle.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        tengfei@...eaurora.org
Subject: Re: [PATCH] arm64: break while loop if task had been rescheduled

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@...eaurora.org>
>> ---
>>  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
>> 

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