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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@redhat.com>
AuthorDate: Fri, 30 Dec 2022 23:11:20 -05:00
Committer: Ingo Molnar <mingo@kernel.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@infradead.org>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Peter Zijlstra <peterz@infradead.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;
}
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