Re: [PATCH] sched: Fix NULL user_cpus_ptr check in dup_user_cpus_ptr()

From: Waiman Long
Date: Wed Nov 23 2022 - 08:09:34 EST



On 11/23/22 06:20, Peter Zijlstra wrote:
On Tue, Nov 22, 2022 at 02:06:53PM -0500, Waiman Long wrote:
In general, a non-null user_cpus_ptr will remain set until the task dies.
A possible exception to this is the fact that do_set_cpus_allowed()
will clear a non-null user_cpus_ptr. To allow this possible racing
condition, we need to check for NULL user_cpus_ptr under the pi_lock
before duping the user mask.

Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
kernel/sched/core.c | 23 +++++++++++++++++++++--
1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8df51b08bb38..f447a6285ea2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2625,7 +2625,14 @@ int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
int node)
{
unsigned long flags;
+ cpumask_t *user_mask = NULL;
The inverse xmas tree is sad :-(
Right. The inverse xmas tree rule. Will fix that.

+ /*
+ * If there is a concurrent sched_setaffinity(), we may miss the
+ * newly updated user_cpus_ptr. However, a non-NULL user_cpus_ptr
+ * is relatively unlikely and it is not worth the extra overhead
+ * of taking the pi_lock on every fork/clone.
+ */
I think the correct argument is saying the thing is racy and loosing the
race is a valid situation. IOW, this is the same as if the concurrent
sched_setaffinity() happens after fork().
Good point, will update the comment.
if (!src->user_cpus_ptr)
return 0;
@@ -2633,10 +2640,22 @@ int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
if (!dst->user_cpus_ptr)
return -ENOMEM;
- /* Use pi_lock to protect content of user_cpus_ptr */
+ /*
+ * Use pi_lock to protect content of user_cpus_ptr
+ *
+ * Though unlikely, user_cpus_ptr can be reset to NULL by a concurrent
+ * do_set_cpus_allowed().
+ */
raw_spin_lock_irqsave(&src->pi_lock, flags);
+ if (src->user_cpus_ptr)
+ cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
+ else
+ swap(dst->user_cpus_ptr, user_mask);
Uhhhh, did you mean to write:

if (src->user_cpus_ptr) {
cpumask_copy(user_mask, src->user_cpus_ptr);
swap(dst->user_cpus_ptr, user_mask);
}

?

Not really. The point is that dst->user_cpus_ptr has been allocated. If src->user_cpus_ptr turns out to be NULL, we need to clear dst->user_cpus_ptr which is what the swap() does and then free that memory after unlock. Will add a comment to make this point clear.

Cheers,
Longman