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

[Peter Zijlstra]

On Thu, Apr 24, 2025 at 12:25:40PM -0400, Steven Rostedt wrote:
> From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> 
> Simplify the get_perf_callchain() user logic a bit.  task_pt_regs()
> should never be NULL.
> 
> Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> ---
>  kernel/events/callchain.c | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
> index abf258913ab6..bba7f8540ade 100644
> --- a/kernel/events/callchain.c
> +++ 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.

>  		}
>  
> -		if (regs) {
> -			if (add_mark)
> -				perf_callchain_store_context(&ctx, PERF_CONTEXT_USER);
> +		if (add_mark)
> +			perf_callchain_store_context(&ctx, PERF_CONTEXT_USER);
>  
> -			start_entry_idx = entry->nr;
> -			perf_callchain_user(&ctx, regs);
> -			fixup_uretprobe_trampoline_entries(entry, start_entry_idx);
> -		}
> +		start_entry_idx = entry->nr;
> +		perf_callchain_user(&ctx, regs);
> +		fixup_uretprobe_trampoline_entries(entry, start_entry_idx);
>  	}
>  
> +exit_put:
>  	put_callchain_entry(rctx);
>  
>  	return entry;
> -- 
> 2.47.2
> 
>