Re: [PATCH v5 11/17] perf: Simplify get_perf_callchain() user logic

On 2025-04-24 13:28, Steven Rostedt wrote:
> On Thu, 24 Apr 2025 18:36:07 +0200
> Peter Zijlstra <[email protected]> wrote:
> 
>>> +++ b/kernel/events/callchain.c
>>> @@ -246,22 +246,20 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
>>>   
>>>   	if (user) {
>>>   		if (!user_mode(regs)) {
>>> -			if  (current->mm)
>>> -				regs = task_pt_regs(current);
>>> -			else
>>> -				regs = NULL;
>>> +			if (!current->mm)
>>> +				goto exit_put;
>>> +			regs = task_pt_regs(current);
>>
>> I'm thinking this might be one of those is-kthread test written as
>> has-mm, and they're broken.
>>
>> Notably things like the io-uring kthreads do have mm.

Can a kthread such as io-uring have user_mode(regs) == true ?

> 
> Would there ever be a case where:
> 
> 	current->mm == NULL && !(current->flags & PF_KTHREAD)
> 
> ?
> 
> That is, do we still need to check for current->mm if it's not a kernel
> thread, or can we assume it exists?

The case I'd be careful about is if the code can nest over exit_mm()
(e.g. interrupt) after it sets current->mm = NULL.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com