Commit 9a5418bc authored by Waiman Long's avatar Waiman Long Committed by Ingo Molnar
Browse files

sched/core: Use kfree_rcu() in do_set_cpus_allowed() 



Commit 851a723e ("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.

Since user_cpus_ptr is not being used in non-SMP configs, the newly
introduced alloc_user_cpus_ptr() helper will return NULL in this case
and sched_setaffinity() is modified to handle this special case.

Fixes: 851a723e ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
Suggested-by: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarWaiman Long <longman@redhat.com>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Reviewed-by: default avatarPeter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20221231041120.440785-3-longman@redhat.com
parent 87ca4f9e

kernel/sched/core.c

+29 −4
Original line number Diff line number Diff line
@@ -2604,9 +2604,29 @@ 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);

}



static cpumask_t *alloc_user_cpus_ptr(int node)

{

	/*

	 * See do_set_cpus_allowed() above for the rcu_head usage.

	 */

	int size = max_t(int, cpumask_size(), sizeof(struct rcu_head));



	return kmalloc_node(size, GFP_KERNEL, node);

}



int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
@@ -2629,7 +2649,7 @@ int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src, 
	if (data_race(!src->user_cpus_ptr))

		return 0;



	user_mask = kmalloc_node(cpumask_size(), GFP_KERNEL, node);

	user_mask = alloc_user_cpus_ptr(node);

	if (!user_mask)

		return -ENOMEM;
@@ -3605,6 +3625,11 @@ static inline bool rq_has_pinned_tasks(struct rq *rq) 
	return false;

}



static inline cpumask_t *alloc_user_cpus_ptr(int node)

{

	return NULL;

}



#endif /* !CONFIG_SMP */



static void
@@ -8265,8 +8290,8 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask) 
	if (retval)

		goto out_put_task;



	user_mask = kmalloc(cpumask_size(), GFP_KERNEL);

	if (!user_mask) {

	user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);

	if (IS_ENABLED(CONFIG_SMP) && !user_mask) {

		retval = -ENOMEM;

		goto out_put_task;

	}