Re:Re: [PATCH] block, bfq: skip dispatch when selected queue has no next_rq

["李佑鸿 " <dayou5941@163.com>]

At 2026-05-21 17:27:28, "Yu Kuai" <yukuai@fnnas.com> wrote:
>Hi,
>
>在 2026/5/20 17:50, dayou5941@163.com 写道:
>> From: liyouhong <liyouhong@kylinos.cn>
>>
>> bfq_dispatch_rq_from_bfqq() dispatches bfqq->next_rq directly. Add a
>> guard in __bfq_dispatch_request() so that dispatch is skipped if the
>> selected queue unexpectedly has no next request.
>>
>> This keeps behavior unchanged in normal paths while avoiding entering
>> the dispatch helper with a missing next_rq.
>
>Can you explan in which case bfqq can be returned from bfq_select_queue
>with next_rq set to NULL? I didn't find it with a quick review.
>
>Take a look at bfq_dispatch_rq_from_bfqq(), bfqq->next_rq will be dereferenced
>unconditionally, which should lead to kernel crash before this patch if
>next_rq can be NULL.

>

Thanks for the clarification.

I agree there is no demonstrated path for `next_rq == NULL` here, so I'll drop
this patch.

One follow-up question: if `next_rq` is guaranteed non-NULL after
`bfq_select_queue()`, would you consider the `if (rq)` check after
`bfq_dispatch_rq_from_bfqq()` redundant for that path?

Thanks,
liyouhong