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* [PATCH-tip v2] sched: Use kfree_rcu() in do_set_cpus_allowed()
@ 2022-11-29  1:26 Waiman Long
  2022-11-29  1:31 ` Waiman Long
  2022-11-30 13:13 ` kernel test robot
  0 siblings, 2 replies; 3+ messages in thread
From: Waiman Long @ 2022-11-29  1:26 UTC (permalink / raw)
  To: Ingo Molnar, Peter Zijlstra, Juri Lelli, Vincent Guittot,
	Dietmar Eggemann, Steven Rostedt, Ben Segall, Mel Gorman,
	Daniel Bristot de Oliveira
  Cc: Phil Auld, linux-kernel, Waiman Long

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 | 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;
-- 
2.31.1


^ permalink raw reply	[flat|nested] 3+ messages in thread
end of thread, other threads:[~2022-11-30 13:14 UTC | newest]

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2022-11-29  1:26 [PATCH-tip v2] sched: Use kfree_rcu() in do_set_cpus_allowed() Waiman Long
2022-11-29  1:31 ` Waiman Long
2022-11-30 13:13 ` kernel test robot

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