Re: [RFC][PATCH 0/2] Add is_user_thread() and is_kernel_thread() helper functions

Julia Lawall
2025-04-26T10:42:34Z

On Fri, 25 Apr 2025, Andrew Morton wrote:

> On Fri, 25 Apr 2025 16:41:20 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > While working on the deferred stacktrace code, Peter Zijlstra told
> > me to use task->flags & PF_KTHREAD instead of checking task->mm for NULL.
> > This seemed reasonable, but while working on it, as there were several
> > places that check if the task is a kernel thread and other places that
> > check if the task is a user space thread I found it a bit confusing
> > when looking at both:
> >
> > if (task->flags & PF_KTHREAD)
> > and
> > if (!(task->flags & PF_KTHREAD))
> >
> > Where I mixed them up sometimes, and checked for a user space thread when I
> > really wanted to check for a kernel thread. I found these mistakes before
> > sending out my patches, but going back and reviewing the code, I always had
> > to stop and spend a few unnecessary seconds making sure the check was
> > testing that flag correctly.
> >
> > To make this a bit more obvious, I introduced two helper functions:
> >
> > is_user_thread(task)
> > is_kernel_thread(task)
> >
> > which simply test the flag for you. Thus, seeing:
> >
> > if (is_user_thread(task))
> > or
> > if (is_kernel_thread(task))
> >
> > it was very obvious to which test you wanted to make.
>
> Seems sensible. Please consider renaming PF_KTHREAD in order to break
> missed conversion sites.

Maybe:

@r depends on !(file in "include/linux/sched.h")@ // Kees's suggestion
position p;
expression e;
@@

(
e = (PF_KTHREAD | ...)
|
e |= (PF_KTHREAD | ...)
|
PF_KTHREAD@p
)

@script:ocaml@ // change to python if desired
p << r.p;
@@

Printf.printf "%s:%d: Warning: remaining use of PF_KTHREAD\n" (List.hd p).file (List.hd p).line

julia