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sched_ext: Use READ_ONCE() for the read side of dsq->nr update

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scx_bpf_dsq_nr_queued() reads dsq->nr via READ_ONCE() without holding
any lock, making dsq->nr a lock-free concurrently accessed variable.
However, dsq_mod_nr(), the sole writer of dsq->nr, only uses
WRITE_ONCE() on the write side without the matching READ_ONCE() on the
read side:

    WRITE_ONCE(dsq->nr, dsq->nr + delta);
                        ^^^^^^^
                        plain read -- KCSAN data race

The KCSAN documentation requires that if one accessor uses READ_ONCE()
or WRITE_ONCE() on a variable to annotate lock-free access, all other
accesses must also use the appropriate accessor. A plain read on the
right-hand side of WRITE_ONCE() leaves the pair incomplete and will
trigger KCSAN warnings.

Fix by using READ_ONCE() for the read side of the update:

    WRITE_ONCE(dsq->nr, READ_ONCE(dsq->nr) + delta);

This is consistent with scx_bpf_dsq_nr_queued() and makes the
concurrent access annotation complete and KCSAN-clean.

Signed-off-by: zhidao su <suzhidao@xiaomi.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
zhidao su 2026-03-02 17:14:39 +08:00 • committed by Tejun Heo
parent 032e084f0d
commit 9adfcef334
1 changed files with 6 additions and 2 deletions
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8
kernel/sched/ext.c
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@ -976,8 +976,12 @@ static bool scx_dsq_priq_less(struct rb_node *node_a,
static void dsq_mod_nr(struct scx_dispatch_q *dsq, s32 delta)
{
/* scx_bpf_dsq_nr_queued() reads ->nr without locking, use WRITE_ONCE() */
WRITE_ONCE(dsq->nr, dsq->nr + delta);
/*
* scx_bpf_dsq_nr_queued() reads ->nr without locking. Use READ_ONCE()
* on the read side and WRITE_ONCE() on the write side to properly
* annotate the concurrent lockless access and avoid KCSAN warnings.
*/
WRITE_ONCE(dsq->nr, READ_ONCE(dsq->nr) + delta);
}
static void refill_task_slice_dfl(struct scx_sched *sch, struct task_struct *p)