[PATCH] sched/headers: move struct sched_param out of uapi

[Kir Kolyshkin]

Both glibc and musl define struct sched_param in sched.h, while kernel
has it in uapi/linux/sched/types.h, making it cumbersome to use
sched_getattr(2) or sched_setattr(2) from userspace.

For example, something like this:

	#include <sched.h>
	#include <linux/sched/types.h>

	struct sched_attr sa;

will result in "error: redefinition of ‘struct sched_param’" (note the
code doesn't need sched_param at all -- it needs struct sched_attr
plus some stuff from sched.h).

The situation is, glibc is not going to provide a wrapper for
sched_{get,set}attr, thus the need to include linux/sched_types.h
directly, which leads to the above problem.

Thus, the userspace is left with a few sub-par choices when it wants to
use e.g. sched_setattr(2), such as maintaining a copy of struct
sched_attr definition, or using some other ugly tricks.

OTOH, struct sched_param is well known, defined in POSIX, and it won't
be ever changed (as that would break backward compatibility).

So, while struct sched_param is indeed part of the kernel uapi,
exposing it the way it's done now creates an issue, and hiding it
(like this patch does) fixes that issue, hopefully without creating
another one: common userspace software rely on libc headers, and as
for "special" software (like libc), it looks like glibc and musl
do not rely on kernel headers for struct sched_param definition
(but let's Cc their mailing lists in case it's otherwise).

The alternative to this patch would be to move struct sched_attr to,
say, linux/sched.h, or linux/sched/attr.h (the new file).

Oh, and here is the previous attempt to fix the issue:
https://lore.kernel.org/all/20200528135552.GA87103@xxxxxxxxxx/
While I support Linus arguments, the issue is still here
and needs to be fixed.

Cc: libc-alpha@xxxxxxxxxxxxxx
Cc: musl@xxxxxxxxxxxxxxxxxx
Cc: linux-api@xxxxxxxxxxxxxxx
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Fixes: e2d1e2aec572 ("sched/headers: Move various ABI definitions to <uapi/linux/sched/types.h>")
Signed-off-by: Kir Kolyshkin <kolyshkin@xxxxxxxxx>
---
 include/linux/sched.h            | 5 ++++-
 include/uapi/linux/sched/types.h | 4 ----
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 609bde814cb0..3167e97a6b04 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -63,7 +63,6 @@ struct robust_list_head;
 struct root_domain;
 struct rq;
 struct sched_attr;
-struct sched_param;
 struct seq_file;
 struct sighand_struct;
 struct signal_struct;
@@ -370,6 +369,10 @@ extern struct root_domain def_root_domain;
 extern struct mutex sched_domains_mutex;
 #endif
 
+struct sched_param {
+	int sched_priority;
+};
+
 struct sched_info {
 #ifdef CONFIG_SCHED_INFO
 	/* Cumulative counters: */
diff --git a/include/uapi/linux/sched/types.h b/include/uapi/linux/sched/types.h
index f2c4589d4dbf..90662385689b 100644
--- a/include/uapi/linux/sched/types.h
+++ b/include/uapi/linux/sched/types.h
@@ -4,10 +4,6 @@
 
 #include <linux/types.h>
 
-struct sched_param {
-	int sched_priority;
-};
-
 #define SCHED_ATTR_SIZE_VER0	48	/* sizeof first published struct */
 #define SCHED_ATTR_SIZE_VER1	56	/* add: util_{min,max} */
 
-- 
2.41.0