mm/slab: disallow kfree_rcu_sheaf() on PREEMPT_RT again

This partially reverts commit 2a8bb29ec9b2 ("mm/slab: allow
kfree_rcu_sheaf() on PREEMPT_RT"). It was based on the assumption that
local_trylock() is safe on PREEMPT_RT from any context.

However kvfree_rcu() is also called by set_cpus_allowed_force() with
task_struct::pi_lock acquired and there it's not safe, as syzbot
has reported.

For the immediate fix, skip kfree_rcu_sheaf() on PREEMPT_RT again from
kvfree_call_rcu(). In theory, kfree_rcu_nolock() would have the same
problem when called from under pi_lock on PREEMPT_RT but that can
be addressed if such a caller is proposed.

Add an explanation comment, courtesy of Sebastian.

Reported-by: syzbot+acf142088e0182172e58@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=acf142088e0182172e58
Reported-by: ThangNN99 <ngocthang2710.1999@gmail.com>
Fixes: 2a8bb29ec9b2 ("mm/slab: allow kfree_rcu_sheaf() on PREEMPT_RT")
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20260831-b4-kfree_rcu_hotfix-v1-1-4f0fb882638b@kernel.org
Reviewed-by: Harry Yoo (Meta) <harry@kernel.org>
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

0 files changed, 0 insertions, 0 deletions