[PATCH-tip] sched: Don't call kfree() in do_set_cpus_allowed()

[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 not be in a context where kfree() can be safely called. So the
following splats may be printed:

   WARNING: possible circular locking dependency detected
   BUG: sleeping function called from invalid context

To avoid these problems without leaking memory, the free cpumask is now
put into a lockless list to be reused in a later sched_setaffinity()
call instead.

Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 kernel/sched/core.c | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 78b2d5cabcc5..8df51b08bb38 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2527,6 +2527,11 @@ int push_cpu_stop(void *arg)
 	return 0;
 }
 
+/*
+ * A lockless list of user cpumask available to be reused.
+ */
+static LLIST_HEAD(free_cpumasks);
+
 /*
  * sched_class::set_cpus_allowed must do the below, but is not required to
  * actually call this function.
@@ -2606,7 +2611,14 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
 	};
 
 	__do_set_cpus_allowed(p, &ac);
-	kfree(ac.user_mask);
+	if (ac.user_mask) {
+		/*
+		 * We may not be in a context where kfree() can be called.
+		 * Put the free user_mask in free_cpumasks to be freed or
+		 * used later.
+		 */
+		llist_add((struct llist_node *)ac.user_mask, &free_cpumasks);
+	}
 }
 
 int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
@@ -8194,7 +8206,7 @@ __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx)
 long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
 {
 	struct affinity_context ac;
-	struct cpumask *user_mask;
+	struct cpumask *user_mask = NULL;
 	struct task_struct *p;
 	int retval;
 
@@ -8229,7 +8241,15 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
 	if (retval)
 		goto out_put_task;
 
-	user_mask = kmalloc(cpumask_size(), GFP_KERNEL);
+	/*
+	 * Use the element in the free_cpumasks, if available.
+	 */
+	if (!llist_empty(&free_cpumasks))
+		user_mask = (struct cpumask *)llist_del_first(&free_cpumasks);
+
+	if (!user_mask)
+		user_mask = kmalloc(cpumask_size(), GFP_KERNEL);
+
 	if (!user_mask) {
 		retval = -ENOMEM;
 		goto out_put_task;
-- 
2.31.1