Messages in this thread | | | Date | Tue, 22 Nov 2022 08:33:49 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH-tip] sched: Don't call kfree() in do_set_cpus_allowed() |
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On Tue, Nov 22, 2022 at 10:23:43AM -0500, Waiman Long wrote: > 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.
Indeed, the NULL check is in kvfree_rcu_arg_2(). By the time you get to kvfree_call_rcu, the pointer is assumed to be non-NULL.
> > 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);
This looks plausible to me.
Thanx, Paul
> } > > 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 > >
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