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SubjectRe: [PATCH-tip v3] sched: Use kfree_rcu() in do_set_cpus_allowed()
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On 12/22/22 14:32, Peter Zijlstra wrote:
> On Mon, Dec 05, 2022 at 11:39:36AM -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()
>> 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@infradead.org>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>> kernel/sched/core.c | 20 +++++++++++++++++---
>> 1 file changed, 17 insertions(+), 3 deletions(-)
>>
>> [v3: Fix build problem reported by kernel test robot]
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 57e5932f81a9..155b6cfe119a 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2604,9 +2604,19 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
>> .user_mask = NULL,
>> .flags = SCA_USER, /* clear the user requested mask */
>> };
>> + union cpumask_rcuhead {
>> + cpumask_t cpumask;
>> + struct rcu_head rcu;
>> + };
>>
>> __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 +8230,7 @@ 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 +8263,11 @@ 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;
> AFAICT you forgot dup_user_cpus_ptr().

I haven't received any response from you for a while. So it is just a
ping. Of course, I am aware that there is another dup_user_cpus_ptr()
patch ouststanding. I will of course talk about that when you respond. I
also have a pending rwsem patch series waiting for your review:-)

Cheers,
Longman

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