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

Steven Rostedt
2025-04-25T20:41:15Z

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.

I then created a coccinelle script to change all the checks throughout the
kernel to use one of these macros.

$ cat kthread.cocci
@@
identifier task;
@@
- !(task->flags & PF_KTHREAD)
+ is_user_thread(task)
@@
identifier task;
@@
- (task->flags & PF_KTHREAD) == 0
+ is_user_thread(task)
@@
identifier task;
@@
- (task->flags & PF_KTHREAD) != 0
+ is_kernel_thread(task)
@@
identifier task;
@@
- task->flags & PF_KTHREAD
+ is_kernel_thread(task)

$ spatch --dir --include-headers kthread.cocci . > /tmp/t.patch
$ patch -p1 < /tmp/t.patch

Make sure to undo the conversion of the helper functions themselves!

$ git show include/linux/sched.h | patch -p1 -R

It did modify the tools/sched_ext code, and I'm not sure if that's OK
or not. Does it still use the sched.h header? If so, it should be fine.
But if it is an issue, I can undo the changes to tools as well.

Steven Rostedt (2):
kthread: Add is_user_thread() and is_kernel_thread() helper functions
treewide: Have the task->flags & PF_KTHREAD check use the helper functions

----
arch/arm/mm/init.c | 2 +-
arch/arm64/include/asm/uaccess.h | 2 +-
arch/arm64/kernel/process.c | 2 +-
arch/arm64/kernel/proton-pack.c | 2 +-
arch/mips/kernel/process.c | 2 +-
arch/powerpc/kernel/process.c | 2 +-
arch/powerpc/kernel/stacktrace.c | 2 +-
arch/x86/kernel/fpu/core.c | 2 +-
arch/x86/kernel/process.c | 2 +-
block/blk-cgroup.c | 2 +-
drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd.h | 2 +-
drivers/md/dm-vdo/logger.c | 2 +-
drivers/tty/sysrq.c | 2 +-
fs/bcachefs/clock.c | 2 +-
fs/bcachefs/journal_reclaim.c | 2 +-
fs/bcachefs/move.c | 6 +++---
fs/exec.c | 2 +-
fs/file_table.c | 2 +-
fs/namespace.c | 2 +-
fs/proc/array.c | 4 ++--
fs/proc/base.c | 6 +++---
include/linux/sched.h | 10 ++++++++++
io_uring/io_uring.c | 2 +-
kernel/cgroup/cgroup.c | 6 +++---
kernel/cgroup/freezer.c | 4 ++--
kernel/events/core.c | 2 +-
kernel/exit.c | 2 +-
kernel/fork.c | 6 +++---
kernel/freezer.c | 4 ++--
kernel/futex/pi.c | 2 +-
kernel/kthread.c | 12 ++++++------
kernel/livepatch/transition.c | 2 +-
kernel/power/process.c | 2 +-
kernel/sched/core.c | 6 +++---
kernel/sched/idle.c | 2 +-
kernel/sched/sched.h | 4 ++--
kernel/signal.c | 4 ++--
kernel/stacktrace.c | 2 +-
lib/is_single_threaded.c | 2 +-
mm/memcontrol.c | 2 +-
mm/oom_kill.c | 4 ++--
mm/page_alloc.c | 2 +-
mm/vmscan.c | 2 +-
security/keys/request_key.c | 2 +-
security/smack/smack_access.c | 2 +-
security/smack/smack_lsm.c | 4 ++--
security/tomoyo/network.c | 2 +-
security/yama/yama_lsm.c | 2 +-
tools/sched_ext/scx_central.bpf.c | 2 +-
tools/sched_ext/scx_flatcg.bpf.c | 2 +-
tools/sched_ext/scx_qmap.bpf.c | 2 +-
51 files changed, 82 insertions(+), 72 deletions(-)