linux-kernel - Re: [RFC PATCH v2 2/2] blk-mq: Lockout tagset iter when freeing rqs

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Date:   Tue, 22 Dec 2020 08:16:59 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     John Garry <john.garry@...wei.com>, axboe@...nel.dk,
        ming.lei@...hat.com
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        hch@....de, hare@...e.de, ppvk@...eaurora.org,
        kashyap.desai@...adcom.com, linuxarm@...wei.com
Subject: Re: [RFC PATCH v2 2/2] blk-mq: Lockout tagset iter when freeing rqs

On 12/22/20 3:15 AM, John Garry wrote:
> So then we could have something like this:
> 
> ---8<---
> 
>   -435,9 +444,13 @@ void blk_mq_queue_tag_busy_iter(struct request_queue 
> *q, busy_iter_fn *fn,
>      if (!blk_mq_hw_queue_mapped(hctx))
>              continue;
> 
> +    while (!atomic_inc_not_zero(&tags->iter_usage_counter));
> +
>      if (tags->nr_reserved_tags)
>          bt_for_each(hctx, tags->breserved_tags, fn, priv, true);
>      bt_for_each(hctx, tags->bitmap_tags, fn, priv, false);
> 
> +    atomic_dec(&tags->iter_usage_counter);
> }
> 
> blk_queue_exit(q);
> 
> --->8---
> 
> And similar for blk_mq_tagset_busy_iter(). How about it?

Are there any blk_mq_tagset_busy_iter() calls that happen from a context 
where the tag set can disappear while that function is in progress?

Some blk_mq_tagset_busy_iter() calls happen from a context where it is 
not allowed to sleep but also where it is guaranteed that the tag set 
won't disappear, e.g. the call from inside sdk_mq_queue_rq().

How about using a mutex inside blk_mq_queue_tag_busy_iter() instead? As 
far as I can see all blk_mq_queue_tag_busy_iter() happen from a context 
where it is allowed to sleep.

Thanks,

Bart.