Re: [PATCH v11 1/5] arm64: Call stack_backtrace() only from within walk_stackframe()

["Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>]

Hey Mark,

Do you have any comments on the rest of the series? I am working on the next version of the patchset.
If you have any other comments, I will wait.

Thanks.

Madhavan

On 11/30/21 2:29 PM, Madhavan T. Venkataraman wrote:
> 
> 
> On 11/30/21 12:29 PM, Mark Rutland wrote:
>> On Tue, Nov 30, 2021 at 11:13:28AM -0600, Madhavan T. Venkataraman wrote:
>>> On 11/30/21 9:05 AM, Mark Rutland wrote:
>>>> On Tue, Nov 23, 2021 at 01:37:19PM -0600, madvenka@linux.microsoft.com wrote:
>>>>> From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
>>>>>
>>>>> Currently, arch_stack_walk() calls start_backtrace() and walk_stackframe()
>>>>> separately. There is no need to do that. Instead, call start_backtrace()
>>>>> from within walk_stackframe(). In other words, walk_stackframe() is the only
>>>>> unwind function a consumer needs to call.
>>
>>>>> @@ -143,15 +140,19 @@ static int notrace unwind_frame(struct task_struct *tsk,
>>>>>  NOKPROBE_SYMBOL(unwind_frame);
>>>>>  
>>>>>  static void notrace walk_stackframe(struct task_struct *tsk,
>>>>> -				    struct stackframe *frame,
>>>>> +				    unsigned long fp, unsigned long pc,
>>>>>  				    bool (*fn)(void *, unsigned long), void *data)
>>>>>  {
>>>>> +	struct stackframe frame;
>>>>> +
>>>>> +	start_backtrace(&frame, fp, pc);
>>>>> +
>>>>>  	while (1) {
>>>>>  		int ret;
>>>>>  
>>>>> -		if (!fn(data, frame->pc))
>>>>> +		if (!fn(data, frame.pc))
>>>>>  			break;
>>>>> -		ret = unwind_frame(tsk, frame);
>>>>> +		ret = unwind_frame(tsk, &frame);
>>>>>  		if (ret < 0)
>>>>>  			break;
>>>>>  	}
>>>>> @@ -195,17 +196,19 @@ noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
>>>>>  			      void *cookie, struct task_struct *task,
>>>>>  			      struct pt_regs *regs)
>>>>>  {
>>>>> -	struct stackframe frame;
>>>>> -
>>>>> -	if (regs)
>>>>> -		start_backtrace(&frame, regs->regs[29], regs->pc);
>>>>> -	else if (task == current)
>>>>> -		start_backtrace(&frame,
>>>>> -				(unsigned long)__builtin_frame_address(1),
>>>>> -				(unsigned long)__builtin_return_address(0));
>>>>> -	else
>>>>> -		start_backtrace(&frame, thread_saved_fp(task),
>>>>> -				thread_saved_pc(task));
>>>>> -
>>>>> -	walk_stackframe(task, &frame, consume_entry, cookie);
>>>>> +	unsigned long fp, pc;
>>>>> +
>>>>> +	if (regs) {
>>>>> +		fp = regs->regs[29];
>>>>> +		pc = regs->pc;
>>>>> +	} else if (task == current) {
>>>>> +		/* Skip arch_stack_walk() in the stack trace. */
>>>>> +		fp = (unsigned long)__builtin_frame_address(1);
>>>>> +		pc = (unsigned long)__builtin_return_address(0);
>>>>> +	} else {
>>>>> +		/* Caller guarantees that the task is not running. */
>>>>> +		fp = thread_saved_fp(task);
>>>>> +		pc = thread_saved_pc(task);
>>>>> +	}
>>>>> +	walk_stackframe(task, fp, pc, consume_entry, cookie);
>>>>
>>>> I'd prefer to leave this as-is. The new and old structure are largely
>>>> equivalent, so we haven't made this any simpler, but we have added more
>>>> arguments to walk_stackframe().
>>>>
>>>
>>> This is just to simplify things when we eventually add arch_stack_walk_reliable().
>>> That is all. All of the unwinding is done by a single unwinding function and
>>> there are two consumers of that unwinding function - arch_stack_walk() and
>>> arch_stack_walk_reliable().
>>
>> I understand the theory, but I don't think that moving the start_backtrace()
>> call actually simplifies this in a meaningful way, and I think it'll make it
>> harder for us to make more meaningful simplifications later on.
>>
>> As of patch 4 of this series, we'll have:
>>
>> | noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
>> | 				      void *cookie, struct task_struct *task,
>> | 				      struct pt_regs *regs)
>> | {
>> | 	unsigned long fp, pc;
>> | 
>> | 	if (regs) {
>> | 		fp = regs->regs[29];
>> | 		pc = regs->pc;
>> | 	} else if (task == current) {
>> | 		/* Skip arch_stack_walk() in the stack trace. */
>> | 		fp = (unsigned long)__builtin_frame_address(1);
>> | 		pc = (unsigned long)__builtin_return_address(0);
>> | 	} else {
>> | 		/* Caller guarantees that the task is not running. */
>> | 		fp = thread_saved_fp(task);
>> | 		pc = thread_saved_pc(task);
>> | 	}
>> | 	walk_stackframe(task, fp, pc, consume_entry, cookie);
>> | }
>> | 
>> | noinline int notrace arch_stack_walk_reliable(stack_trace_consume_fn consume_fn,
>> |                                              void *cookie,
>> |                                              struct task_struct *task)
>> | {
>> | 	unsigned long fp, pc;
>> | 
>> | 	if (task == current) {
>> | 		/* Skip arch_stack_walk_reliable() in the stack trace. */
>> | 		fp = (unsigned long)__builtin_frame_address(1);
>> | 		pc = (unsigned long)__builtin_return_address(0);
>> | 	} else {
>> | 		/* Caller guarantees that the task is not running. */
>> | 		fp = thread_saved_fp(task);
>> | 		pc = thread_saved_pc(task);
>> | 	}
>> | 	if (unwind(task, fp, pc, consume_fn, cookie))
>> | 		return 0;
>> | 	return -EINVAL;
>> | }
>>
>> Which I do not think is substantially simpler than the naive extrapolation from
>> what we currently have, e.g.
>>
>> | noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
>> | 				      void *cookie, struct task_struct *task,
>> | 				      struct pt_regs *regs)
>> | {
>> |	struct stackframe frame;
>> | 
>> | 	if (regs) {
>> |		unwind_init(&frame, regs->regs[29], regs->pc)
>> | 	} else if (task == current) {
>> |		unwind_init(&frame, __builtin_frame_address(1),
>> |			    __builtin_return_address(0));
>> | 	} else {
>> |		unwind_init(&frame, thread_saved_fp(task),
>> |			    thread_saved_pc(task);
>> | 	}
>> | 	walk_stackframe(task, &frame, consume_entry, cookie);
>> | }
>> | 
>> | noinline int notrace arch_stack_walk_reliable(stack_trace_consume_fn consume_fn,
>> |                                              void *cookie,
>> |                                              struct task_struct *task)
>> | {
>> |	struct stackframe frame;
>> | 
>> | 	if (task == current) {
>> |		unwind_init(&frame, __builtin_frame_address(1),
>> |			    __builtin_return_address(0));
>> | 	} else {
>> |		unwind_init(&frame, thread_saved_fp(task),
>> |			    thread_saved_pc(task);
>> | 	}
>> | 	if (unwind(task, &frame, consume_fn, cookie))
>> | 		return 0;
>> | 	return -EINVAL;
>> | }
>>
>> Further, I think we can factor this in a different way to reduce the
>> duplication:
>>
>> | /*
>> |  * TODO: document requirements here
>> |  */
>> | static inline void unwind_init_from_current_regs(struct stackframe *frame,
>> | 						 struct pt_regs *regs)
>> | {
>> | 	unwind_init(frame, regs->regs[29], regs->pc);
>> | }
>> | 
>> | /*
>> |  * TODO: document requirements here
>> |  */
>> | static inline void unwind_init_from_blocked_task(struct stackframe *frame,
>> | 						 struct task_struct *tsk)
>> | {
>> | 	unwind_init(&frame, thread_saved_fp(task),
>> | 		    thread_saved_pc(task));
>> | }
>> | 
>> | /*
>> |  * TODO: document requirements here
>> |  *
>> |  * Note: this is always inlined, and we expect our caller to be a noinline
>> |  * function, such that this starts from our caller's caller.
>> |  */
>> | static __always_inline void unwind_init_from_caller(struct stackframe *frame)
>> | {
>> | 	unwind_init(frame, __builtin_frame_address(1),
>> | 		    __builtin_return_address(0));
>> | }
>> |
>> | noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
>> | 				      void *cookie, struct task_struct *task,
>> | 				      struct pt_regs *regs)
>> | {
>> |	struct stackframe frame;
>> | 
>> | 	if (regs)
>> |		unwind_init_current_regs(&frame, regs);
>> |	else if (task == current)
>> |		unwind_init_from_caller(&frame);
>> |	else
>> |		unwind_init_blocked_task(&frame, task);
>> |
>> |	unwind(task, &frame, consume_entry, cookie);
>> | }
>> |
>> | noinline int notrace arch_stack_walk_reliable(stack_trace_consume_fn consume_fn,
>> |                                              void *cookie,
>> |                                              struct task_struct *task)
>> | {
>> |	struct stackframe frame;
>> | 
>> | 	if (task == current)
>> |		unwind_init_from_caller(&frame);
>> | 	else
>> |		unwind_init_from_blocked_task(&frame, task);
>> |
>> | 	if (unwind(task, &frame, consume_fn, cookie))
>> | 		return 0;
>> | 	return -EINVAL;
>> | }
>>
>> ... which minimizes the duplication and allows us to add specialized
>> initialization for each case if necessary, which I believe we will need in
>> future to make unwinding across exception boundaries (such as when starting
>> with regs) more useful.
>>
>> Thanks,
>> Mark.
>>
> 
> OK. I don't mind doing it this way.
> 
> Thanks.
> 
> Madhavan
>