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Message-ID: <5ff3e87b-b8ce-4836-e95b-f1cbe2f5f253@redhat.com>
Date: Mon, 28 Nov 2022 20:31:08 -0500
From: Waiman Long <longman@...hat.com>
To: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>
Cc: Phil Auld <pauld@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH-tip v2] sched: Use kfree_rcu() in do_set_cpus_allowed()
On 11/28/22 20:26, 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()
> may have pi_lock held when calling it. So the following splats may be
> printed especially when running with a PREEMPT_RT kernel:
>
> WARNING: possible circular locking dependency detected
> BUG: sleeping function called from invalid context
>
> To avoid these problems, kfree_rcu() is used instead. An internal
> cpumask_rcuhead union is created for the sole purpose of facilitating
> the use of kfree_rcu() to free the cpumask.
>
> Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
> kernel/sched/core.c | 21 ++++++++++++++++++---
> 1 file changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f7c5253bfd21..bf9066b4f481 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 +2611,13 @@ 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,
> @@ -8220,7 +8231,7 @@ 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();
>
> @@ -8253,7 +8264,11 @@ 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;
Peter, I would like to get this done before the next merge window. I
personally prefer to define a useless union to facilitate the use of
kfree_rcu(). I can also live without it and call kvfree_call_rcu()
directly if this is what you want.
Cheers,
Longman
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