[tip: sched/core] sched: fix warning in sched_setaffinity

[tip-bot2 for Josh Don]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     70ee7947a29029736a1a06c73a48ff37674a851b
Gitweb:        https://git.kernel.org/tip/70ee7947a29029736a1a06c73a48ff37674a851b
Author:        Josh Don <joshdon@xxxxxxxxxx>
AuthorDate:    Mon, 11 Nov 2024 10:27:38 -08:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 02 Dec 2024 12:01:27 +01:00

sched: fix warning in sched_setaffinity

Commit 8f9ea86fdf99b added some logic to sched_setaffinity that included
a WARN when a per-task affinity assignment races with a cpuset update.

Specifically, we can have a race where a cpuset update results in the
task affinity no longer being a subset of the cpuset. That's fine; we
have a fallback to instead use the cpuset mask. However, we have a WARN
set up that will trigger if the cpuset mask has no overlap at all with
the requested task affinity. This shouldn't be a warning condition; its
trivial to create this condition.

Reproduced the warning by the following setup:

- $PID inside a cpuset cgroup
- another thread repeatedly switching the cpuset cpus from 1-2 to just 1
- another thread repeatedly setting the $PID affinity (via taskset) to 2

Fixes: 8f9ea86fdf99b ("sched: Always preserve the user requested cpumask")
Signed-off-by: Josh Don <joshdon@xxxxxxxxxx>
Acked-and-tested-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Waiman Long <longman@xxxxxxxxxx>
Tested-by: Madadi Vineeth Reddy <vineethr@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20241111182738.1832953-1-joshdon@xxxxxxxxxx
---
 kernel/sched/syscalls.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index 0d71fcb..ff0e5ab 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -1200,7 +1200,7 @@ int __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx)
 			bool empty = !cpumask_and(new_mask, new_mask,
 						  ctx->user_mask);
 
-			if (WARN_ON_ONCE(empty))
+			if (empty)
 				cpumask_copy(new_mask, cpus_allowed);
 		}
 		__set_cpus_allowed_ptr(p, ctx);