linux-kernel - Re: [PATCH v3 1/7] blk-mq: avoid access hctx->tags->cpumask before allocation

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Message-ID: <CACVXFVNi_US+e=YJ=n-6PFrv5XN+Q3Zfx5c0MbrAOL6BvSMRfA@mail.gmail.com>
Date:	Sun, 19 Jul 2015 18:24:16 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Keith Busch <keith.busch@...el.com>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH v3 1/7] blk-mq: avoid access hctx->tags->cpumask before allocation

On Sun, Jul 19, 2015 at 12:28 AM, Akinobu Mita <akinobu.mita@...il.com> wrote:
> When unmapped hw queue is remapped after CPU topology is changed,
> hctx->tags->cpumask is set before hctx->tags is allocated in
> blk_mq_map_swqueue().
>
> In order to fix this null pointer dereference, hctx->tags must be
> allocated before configuring hctx->tags->cpumask.

The root cause should be that the mapping between hctx and ctx
can be changed after CPU topo is changed, then hctx->tags can
be changed too, so hctx->tags->cpumask has to be set after
hctx->tags is setup.

>
> Fixes: f26cdc8536 ("blk-mq: Shared tag enhancements")

I am wondering if the above commit considers CPU hotplug, and
nvme uses tag->cpumask to set irq affinity hint just during
starting queue. Looks it should be reasonalbe to
introduce one callback of mapping_changed() for handling
this kind of stuff. But this isn't related with this patch.

> Signed-off-by: Akinobu Mita <akinobu.mita@...il.com>
> Cc: Keith Busch <keith.busch@...el.com>
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Ming Lei <tom.leiming@...il.com>
> ---
>  block/blk-mq.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 7d842db..f29f766 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1821,7 +1821,6 @@ static void blk_mq_map_swqueue(struct request_queue *q)
>
>                 hctx = q->mq_ops->map_queue(q, i);
>                 cpumask_set_cpu(i, hctx->cpumask);
> -               cpumask_set_cpu(i, hctx->tags->cpumask);
>                 ctx->index_hw = hctx->nr_ctx;
>                 hctx->ctxs[hctx->nr_ctx++] = ctx;
>         }
> @@ -1861,6 +1860,14 @@ static void blk_mq_map_swqueue(struct request_queue *q)
>                 hctx->next_cpu = cpumask_first(hctx->cpumask);
>                 hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
>         }
> +
> +       queue_for_each_ctx(q, ctx, i) {
> +               if (!cpu_online(i))
> +                       continue;
> +
> +               hctx = q->mq_ops->map_queue(q, i);
> +               cpumask_set_cpu(i, hctx->tags->cpumask);

If tags->cpumask is always same with hctx->cpumaks, this
CPU iterator can be avoided.

> +       }
>  }
>
>  static void blk_mq_update_tag_set_depth(struct blk_mq_tag_set *set)
> --
> 1.9.1
>



-- 
Ming Lei
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