linux-kernel - [tip: sched/urgent] sched/core: Use kfree_rcu() in do_set_cpus

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Date:   Mon, 09 Jan 2023 10:45:49 -0000
From:   "tip-bot2 for Waiman Long" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: sched/urgent] sched/core: Use kfree_rcu() in do_set_cpus_allowed()

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     9a5418bc48babb313d2a62df29ebe21ce8c06c59
Gitweb:        https://git.kernel.org/tip/9a5418bc48babb313d2a62df29ebe21ce8c06c59
Author:        Waiman Long <longman@...hat.com>
AuthorDate:    Fri, 30 Dec 2022 23:11:20 -05:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Mon, 09 Jan 2023 11:43:23 +01:00

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

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.

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: 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>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Reviewed-by: Peter Zijlstra <peterz@...radead.org>
Link: https://lore.kernel.org/r/20221231041120.440785-3-longman@redhat.com
---
 kernel/sched/core.c | 33 +++++++++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f9f6e54..bb1ee6d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -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;
 	}