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Date:   Tue, 29 Jun 2021 10:25:42 +0200
From:   Daniel Wagner <dwagner@suse.de>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Wen Xiong <wenxiong@us.ibm.com>, james.smart@broadcom.com,
        linux-kernel@vger.kernel.org, sagi@grimberg.me,
        wenxiong@linux.vnet.ibm.com
Subject: Re: [PATCH 1/1] block: System crashes when cpu hotplug + bouncing
 port
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On Tue, Jun 29, 2021 at 11:47:55AM +0800, Ming Lei wrote:
> >    >>data.ctx = __blk_mq_get_ctx(q, cpu);
> >    cpu=2048 if hctx_idx = 4
> 
> Yeah, that is the issue I mentioned, any CPU in hctx->cpumask becomes
> offline, please try the following patch and see if it makes a
> difference:

Given that cpumask_first_and() will return nr_cpu_ids in this case,
can't we just bail out here and have to caller handle the error? I am
able to reproduce the crash you reported in [1] and the fix [2] here
works for me:

--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -469,6 +469,8 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
        if (!blk_mq_hw_queue_mapped(data.hctx))
                goto out_queue_exit;
        cpu = cpumask_first_and(data.hctx->cpumask, cpu_online_mask);
+       if (cpu >= nr_cpu_ids)
+               goto out_queue_exit;
        data.ctx = __blk_mq_get_ctx(q, cpu);
 
        if (!q->elevator)

[1] https://lore.kernel.org/linux-block/20191117041233.GA30615@ming.t460p/
[2] https://lore.kernel.org/linux-block/20210608183339.70609-1-dwagner@suse.de/