Re: [PATCH] Document that PF_KTHREAD _is_ ABI

Am 31.03.21 um 21:23 schrieb Alexey Dobriyan:

On Mon, Mar 22, 2021 at 07:53:10AM +0000, Christoph Hellwig wrote:

quoted

On Sat, Mar 20, 2021 at 10:23:12AM -0700, Andy Lutomirski wrote:

quoted

quoted

https://github.com/systemd/systemd/blob/main/src/basic/process-util.c#L354
src/basic/process-util.c:is_kernel_thread()

Eww.

Could we fix it differently and more permanently by modifying the proc
code to display the values systemd expects?

Yes, do_task_stat needs a mapping from kernel flags to UABI flags.  And
we should already discard everything we think we can from the UABI
now, and only add the ones back that are required to not break
userspace.

Sure we do. Who is going to find all the flags? I found PF_KTHREAD. :^)

More seriously,

/proc/$pid/stat was expanded to include tsk->flags in 0.99.1 (!!!)

Developers kept adding and shuffling flags probably not even realising
what's going on. The last incident happened at 5.10 when PF_IO_WORKER
was exchanged with PF_VCPU for smaller codegen.

With the create_io_thread(), the impact of PF_IO_WORKER becomes more broadly
visible and userspace might start to look at it in order to find the difference
between userspace and kernel io threads. (I also think it should actually be renamed to
PF_IO_THREAD...)

Jens, what do you think about that?

metze