[tip: sched/urgent] sched/mmcid: Handle vfork()/CLONE_VM correctly

[tip-bot2 for Thomas Gleixner]

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

Commit-ID:     28b5a1395036d6c7a6c8034d85ad3d7d365f192c
Gitweb:        https://git.kernel.org/tip/28b5a1395036d6c7a6c8034d85ad3d7d365f192c
Author:        Thomas Gleixner <tglx@xxxxxxxxxx>
AuthorDate:    Tue, 10 Mar 2026 21:28:58 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 11 Mar 2026 12:01:06 +01:00

sched/mmcid: Handle vfork()/CLONE_VM correctly

Matthieu and Jiri reported stalls where a task endlessly loops in
mm_get_cid() when scheduling in.

It turned out that the logic which handles vfork()'ed tasks is broken. It
is invoked when the number of tasks associated to a process is smaller than
the number of MMCID users. It then walks the task list to find the
vfork()'ed task, but accounts all the already processed tasks as well.

If that double processing brings the number of to be handled tasks to 0,
the walk stops and the vfork()'ed task's CID is not fixed up. As a
consequence a subsequent schedule in fails to acquire a (transitional) CID
and the machine stalls.

Cure this by removing the accounting condition and make the fixup always
walk the full task list if it could not find the exact number of users in
the process' thread list.

Fixes: fbd0e71dc370 ("sched/mmcid: Provide CID ownership mode fixup functions")
Closes: https://lore.kernel.org/b24ffcb3-09d5-4e48-9070-0b69bc654281@xxxxxxxxxx
Reported-by: Matthieu Baerts <matttbe@xxxxxxxxxx>
Reported-by: Jiri Slaby <jirislaby@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
Link: https://patch.msgid.link/20260310202526.048657665@xxxxxxxxxx
---
 kernel/sched/core.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d254278..ee59cd1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10670,10 +10670,7 @@ static void mm_cid_do_fixup_tasks_to_cpus(struct mm_struct *mm)
 	for_each_process_thread(p, t) {
 		if (t == current || t->mm != mm)
 			continue;
-		if (mm_cid_fixup_task_to_cpu(t, mm)) {
-			if (--users == 0)
-				return;
-		}
+		mm_cid_fixup_task_to_cpu(t, mm);
 	}
 }