Re: [PATCH-tip] sched: Don't call kfree() in do_set_cpus_allowed()
From: Waiman Long
Date: Tue Nov 22 2022 - 10:24:59 EST
On 11/22/22 07:37, Peter Zijlstra wrote:
On Mon, Nov 21, 2022 at 10:04:33AM -0500, Waiman Long wrote:
On 11/21/22 05:38, Peter Zijlstra wrote:
On Fri, Nov 18, 2022 at 02:33:02PM -0500, Waiman Long wrote:
Commit 851a723e45d1 ("sched: Always clear user_cpus_ptr in
do_set_cpus_allowed()") may call kfree() if user_cpus_ptr was previously
set. Unfortunately, some of the callers of do_set_cpus_allowed()
'some' ? There's only 3 or so, which one triggers this?
It happenned at __kthread_bind_mask() where do_set_cpus_allowed() is called
with pi_lock held.
[ 1084.820105] <TASK>
[ 1084.820110] dump_stack_lvl+0x57/0x81
[ 1084.820117] check_noncircular+0x103/0x120
[ 10[ 1084.820160] lock_acquire+0xba/0x230
[ 1084.820164] ? kfree+0x10f/0x380
[ 1084.820172] ? do_set_cpus_allowed+0x40/0x60
[ 1084.820181] rt_spin_lock+0x27/0xe0
[ 1084.820184] ? kfree+0x10f/0x380
[ 1084.820188] kfree+0x10f/0x380
[ 1084.820195] do_set_cpus_allowed+0x40/0x60
[ 1084.820203] kthread_bind_mask+0x4a/0x70
[ 1084.820211] create_worker+0xfb/0x1a0
[ 1084.820220] worker_thread+0x2e3/0x3c0
[ 1084.820226] ? process_one_work+0x450/0x450
[ 1084.820230] kthread+0x111/0x130
[ 1084.820236] ? kthread_complete_and_exit+0x20/0x20
[ 1084.820244] ret_from_fork+0x22/0x30
[ 1084.820258] </TASK>
[ 1084.820260] BUG: sleeping function called from invalid context at
kernel/locking/spinlock_rt.c:46
It shows up with PREEMPT_RT kernel.
Oh, I see ..
Maybe. One thing that I am not clear about is why user_cpus_ptr is set in
the first place.
Perhaps someone set an affinity on kthreadd ?
But I'm thinking this exact problem is also possible (rather more likely
even) with select_fallback_rq() that too holds pi_lock (which account
for both other users of this function).
Bah.
And the allocation is just the one long in size (for small configs)
which is just enough space for a single linked list like you had.
That is exactly the reason why I use lockless list.
Urgh.
The below is yuck too, and I'm not sure Paul wants us to use
kvfree_call_rcu() without its wrapper.
---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 78b2d5cabcc5..0d0af0fc7fcf 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2606,7 +2606,12 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
};
__do_set_cpus_allowed(p, &ac);
- kfree(ac.user_mask);
+ /*
+ * Because this is called with p->pi_lock held, it is not possible
+ * to use kfree() here (when PREEMPT_RT=y), therefore punt to using
+ * kfree_rcu().
+ */
+ kvfree_call_rcu((struct rcu_head *)ac.user_mask, (rcu_callback_t)0);
}
I guess you need to do a NULL check before calling kvfree_call_rcu() as
I don't think kvfree_call_rcu() does that. Also it is unlikely that we
need to call it.
int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
@@ -8196,7 +8201,7 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
struct affinity_context ac;
struct cpumask *user_mask;
struct task_struct *p;
- int retval;
+ int retval, size;
rcu_read_lock();
@@ -8229,7 +8234,11 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
if (retval)
goto out_put_task;
- user_mask = kmalloc(cpumask_size(), GFP_KERNEL);
+ /*
+ * See do_set_cpus_allowed() for the rcu_head usage.
+ */
+ size = max_t(int, cpumask_size(), sizeof(struct rcu_head));
+ user_mask = kmalloc(size, GFP_KERNEL);
if (!user_mask) {
retval = -ENOMEM;
goto out_put_task;
I guess that will work too. Just like you, I am a bit uneasy to call
into kvfree_call_rcu() directly as it may change in the future. How about
iff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 78b2d5cabcc5..5fac4aa6ac7f 100644
--- a/kernel/sched/core.c
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 78b2d5cabcc5..5fac4aa6ac7f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2593,6 +2593,11 @@ __do_set_cpus_allowed(struct task_struct *p,
struct affinity_context *ctx)
set_next_task(rq, p);
}
+union cpumask_rcuhead {
+ void *cpumask;
+ struct rcu_head rcu;
+};
+
/*
* Used for kthread_bind() and select_fallback_rq(), in both cases the
user
* affinity (if any) should be destroyed too.
@@ -2606,7 +2611,12 @@ void do_set_cpus_allowed(struct task_struct *p,
const struct cpumask *new_mask)
};
__do_set_cpus_allowed(p, &ac);
- kfree(ac.user_mask);
+ /*
+ * Because this is called with p->pi_lock held, it is not possible
+ * to use kfree() here (when PREEMPT_RT=y), therefore punt to using
+ * kfree_rcu().
+ */
+ kfree_rcu((union cpumask_rcuhead *)ac.user_mask, rcu);
}
int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
@@ -8196,7 +8206,7 @@ long sched_setaffinity(pid_t pid, const struct
cpumask *in_mask)
struct affinity_context ac;
struct cpumask *user_mask;
struct task_struct *p;
- int retval;
+ int retval, size;
rcu_read_lock();
@@ -8229,7 +8239,11 @@ long sched_setaffinity(pid_t pid, const struct
cpumask *in_mask)
if (retval)
goto out_put_task;
- user_mask = kmalloc(cpumask_size(), GFP_KERNEL);
+ /*
+ * See do_set_cpus_allowed() for the rcu_head usage.
+ */
+ size = max_t(int, cpumask_size(), sizeof(union cpumask_rcuhead));
+ user_mask = kmalloc(size, GFP_KERNEL);
if (!user_mask) {
retval = -ENOMEM;
goto out_put_task;
Cheers,
Longman