From: Alexey Dobriyan <adobriyan@gmail.com>
To: akpm@linux-foundation.org
Cc: linux-kernel@vger.kernel.org, linux-api@vger.kernel.org
Subject: [PATCH] Document that PF_KTHREAD _is_ ABI
Date: Sat, 20 Mar 2021 19:30:48 +0300 [thread overview]
Message-ID: <YFYjOB1jpbqyNPAp@localhost.localdomain> (raw)
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:
https://github.com/systemd/systemd/blob/main/src/basic/process-util.c#L354
src/basic/process-util.c:is_kernel_thread()
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
another.
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---
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 */
next reply other threads:[~2021-03-20 16:31 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-20 16:30 Alexey Dobriyan [this message]
2021-03-20 17:23 ` [PATCH] Document that PF_KTHREAD _is_ ABI Andy Lutomirski
2021-03-20 17:39 ` Alexey Dobriyan
2021-03-20 19:20 ` Florian Weimer
2021-03-22 7:53 ` Christoph Hellwig
2021-03-31 19:23 ` Alexey Dobriyan
2021-04-01 15:05 ` Stefan Metzmacher
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