Re: [PATCH] Document that PF_KTHREAD _is_ ABI
From: Andy Lutomirski
Date: Sat Mar 20 2021 - 13:24:19 EST
> On Mar 20, 2021, at 9:31 AM, Alexey Dobriyan <adobriyan@xxxxxxxxx> wrote:
> PF_KTHREAD value is visible via field number 9 of /proc/*/stat
> $ sudo cat /proc/2/stat
> 2 (kthreadd) S 0 0 0 0 -1 2129984 0 ...
> It is used by at least systemd to check for kernel-threadness:
Could we fix it differently and more permanently by modifying the proc
code to display the values systemd expects?
> It means that the value can't be changed despite perceived notion that
> task_struct flags are internal to kernel and can be shuffled at whim.
> Formally, _all_ struct task_struct::flags PF_* values are kernel ABI
> which is a disaster.
> I hope we can mask everything but few flags and hope for the best :^)
> Note for beginner Linux programmers:
> every other way you find on the interwebs and Stack Overflow
> for checking if pid belongs to a kernel thread is broken one way or
> Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> include/linux/sched.h | 3 +++
> 1 file changed, 3 insertions(+)
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1566,6 +1566,9 @@ extern struct pid *cad_pid;
> #define PF_MEMALLOC_NOIO 0x00080000 /* All allocation requests will inherit GFP_NOIO */
> #define PF_LOCAL_THROTTLE 0x00100000 /* Throttle writes only against the bdi I write to,
> * I am cleaning dirty pages from some other bdi. */
> + * PF_KTHREAD is part of kernel ABI, visible via value #9 in /proc/$pid/stat
> + */
> #define PF_KTHREAD 0x00200000 /* I am a kernel thread */
> #define PF_RANDOMIZE 0x00400000 /* Randomize virtual address space */
> #define PF_SWAPWRITE 0x00800000 /* Allowed to write to swap */