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sched_ext: Fix scx_bpf_dsq_peek() with FIFO DSQs

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When removing a task from a FIFO DSQ, we must delete it from the list
before updating dsq->first_task, otherwise the following lookup will
just re-read the same task, leaving first_task pointing to removed
entry.

This issue only affects DSQs operating in FIFO mode, as priority DSQs
correctly update the rbtree before re-evaluating the new first task.

Remove the item from the list before refreshing the first task to
guarantee the correct behavior in FIFO DSQs.

Fixes: 44f5c8ec5b ("sched_ext: Add lockless peek operation for DSQs")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Andrea Righi
2025-10-25 00:01:02 +02:00
committed by Tejun Heo
parent e67708823d
commit 71d7847cad
1 changed files with 3 additions and 3 deletions
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6
kernel/sched/ext.c
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@@ -1044,15 +1044,15 @@ static void task_unlink_from_dsq(struct task_struct *p,
p->scx.dsq_flags &= ~SCX_TASK_DSQ_ON_PRIQ;
}
list_del_init(&p->scx.dsq_list.node);
dsq_mod_nr(dsq, -1);
if (!(dsq->id & SCX_DSQ_FLAG_BUILTIN) && dsq->first_task == p) {
struct task_struct *first_task;
first_task = nldsq_next_task(dsq, NULL, false);
rcu_assign_pointer(dsq->first_task, first_task);
}
list_del_init(&p->scx.dsq_list.node);
dsq_mod_nr(dsq, -1);
}
static void dispatch_dequeue(struct rq *rq, struct task_struct *p)