Re: [PATCH bpf v1] bpf: cpumap: fix race in bq_flush_to_queue on PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-02-11 14:44:16 [+0800], Jiayuan Chen wrote:
> From: Jiayuan Chen <jiayuan.chen@shopee.com>
> 
> On PREEMPT_RT kernels, the per-CPU xdp_bulk_queue (bq) can be accessed
> concurrently by multiple preemptible tasks on the same CPU.
> 
> The original code assumes bq_enqueue() and __cpu_map_flush() run
> atomically with respect to each other on the same CPU, relying on
> local_bh_disable() to prevent preemption. However, on PREEMPT_RT,
> local_bh_disable() only calls migrate_disable() and does not disable
> preemption. spin_lock() also becomes a sleeping rt_mutex. Together,
> this allows CFS scheduling to preempt a task during bq_flush_to_queue(),
> enabling another task on the same CPU to enter bq_enqueue() and operate
> on the same per-CPU bq concurrently.
…
> Fixes: d2d6422f8bd1 ("x86: Allow to enable PREEMPT_RT.")

Can you reproduce this? It should not trigger with the commit above.
It should trigger starting with
   3253cb49cbad4 ("softirq: Allow to drop the softirq-BKL lock on PREEMPT_RT")

> Reported-by: syzbot+2b3391f44313b3983e91@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/69369331.a70a0220.38f243.009d.GAE@google.com/T/
> Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>  kernel/bpf/cpumap.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
> index 04171fbc39cb..7fda8421ec40 100644
> --- a/kernel/bpf/cpumap.c
> +++ b/kernel/bpf/cpumap.c
> @@ -714,6 +717,7 @@ const struct bpf_map_ops cpu_map_ops = {
>  	.map_redirect		= cpu_map_redirect,
>  };
>  
> +/* Caller must hold bq->bq_lock */

If this information is important please use lockdep_assert_held() in the
function below. This can be used by lockdep and is understood by humans
while the comment is only visible to humans.

>  static void bq_flush_to_queue(struct xdp_bulk_queue *bq)
>  {
>  	struct bpf_cpu_map_entry *rcpu = bq->obj;
> @@ -750,10 +754,16 @@ static void bq_flush_to_queue(struct xdp_bulk_queue *bq)
>  
>  /* Runs under RCU-read-side, plus in softirq under NAPI protection.
>   * Thus, safe percpu variable access.

+ PREEMPT_RT relies on local_lock_nested_bh().

> + *
> + * On PREEMPT_RT, local_bh_disable() does not disable preemption,
> + * so we use local_lock to serialize access to the per-CPU bq.
>   */
>  static void bq_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_frame *xdpf)
>  {
> -	struct xdp_bulk_queue *bq = this_cpu_ptr(rcpu->bulkq);
> +	struct xdp_bulk_queue *bq;
> +
> +	local_lock(&rcpu->bulkq->bq_lock);

local_lock_nested_bh() & the matching unlock here and in the other
places, please.

Sebastian