This partially reverts commit 2a8bb29ec9b2 ("mm/slab: allow kfree_rcu_sheaf() on PREEMPT_RT"). It was based on an 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 Fixes: 2a8bb29ec9b2 ("mm/slab: allow kfree_rcu_sheaf() on PREEMPT_RT") Signed-off-by: Vlastimil Babka (SUSE) --- Incidentally I have posted a RFC [1] that leads to replacing that kfree_rcu() from set_cpus_allowed_force() but now after back from vacation I need to check the feedback and based on this bug report I can already see it makes the same bad assumption that trylock is fine. [1] https://lore.kernel.org/all/20260807-kfree_nolock_kmalloc-v1-0-ba993cbf7a60@kernel.org/ --- mm/slab_common.c | 18 ++++++++---------- mm/slub.c | 5 +++-- 2 files changed, 11 insertions(+), 12 deletions(-) diff --git a/mm/slab_common.c b/mm/slab_common.c index b19ba1b31484..7223a7596dab 100644 --- a/mm/slab_common.c +++ b/mm/slab_common.c @@ -1667,14 +1667,6 @@ static bool kfree_rcu_sheaf(void *obj) { struct kmem_cache *s; struct slab *slab; - unsigned int free_flags = SLAB_FREE_DEFAULT; - - /* - * It is not safe to spin on PREEMPT_RT because the kernel might be - * holding a raw spinlock and slab acquires sleeping locks. - */ - if (IS_ENABLED(CONFIG_PREEMPT_RT)) - free_flags = SLAB_FREE_NOLOCK; if (is_vmalloc_addr(obj)) return false; @@ -1685,7 +1677,7 @@ static bool kfree_rcu_sheaf(void *obj) s = slab->slab_cache; if (likely(!IS_ENABLED(CONFIG_NUMA) || slab_nid(slab) == numa_mem_id())) - return __kfree_rcu_sheaf(s, obj, free_flags); + return __kfree_rcu_sheaf(s, obj, SLAB_FREE_DEFAULT); return false; } @@ -2034,7 +2026,13 @@ void kvfree_call_rcu(struct kvfree_rcu_head *head, void *ptr) if (!head) might_sleep(); - if (kfree_rcu_sheaf(ptr)) + /* + * kvfree_rcu() is called by set_cpus_allowed_force() with + * task_struct::pi_lock acquired. On PREEMPT_RT the local_trylock() + * usage below will acquire the waitlock which must be avoided. + * Therefore avoid it on PREEMPT_RT. + */ + if (!IS_ENABLED(CONFIG_PREEMPT_RT) && kfree_rcu_sheaf(ptr)) return; // Queue the object but don't yet schedule the batch. diff --git a/mm/slub.c b/mm/slub.c index f9b56cb439e7..7a7e906a0e44 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -6088,8 +6088,9 @@ static void rcu_free_sheaf(struct rcu_head *head) /* * kvfree_call_rcu() can be called while holding a raw_spinlock_t. Since * __kfree_rcu_sheaf() may acquire a spinlock_t (sleeping lock on PREEMPT_RT), - * this would violate lock nesting rules. Therefore, kvfree_call_rcu() avoids - * this problem by passing SLAB_FREE_NOLOCK on PREEMPT_RT. + * this would violate lock nesting rules. Therefore, kfree_call_rcu_nolock() + * avoids this problem by passing SLAB_FREE_NOLOCK. kvfree_call_rcu() is + * bypassing the sheaves layer completely on PREEMPT_RT. * * However, lockdep still complains that it is invalid to acquire spinlock_t * while holding raw_spinlock_t, even on !PREEMPT_RT where spinlock_t is a --- base-commit: cee9395acd8043be0644b25c34bfa86623f2b935 change-id: 20260831-b4-kfree_rcu_hotfix-1d3dd315bff0