Messages in this thread Patch in this message | | | Date | Tue, 22 Nov 2022 13:37:17 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH-tip] sched: Don't call kfree() in do_set_cpus_allowed() |
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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.
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); } 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;
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