Re: [PATCH 8/8] blk-mq: drain I/O when all CPUs in a hctx are offline

From: Bart Van Assche <bvanassche@acm.org>
To: Christoph Hellwig <hch@lst.de>
Cc: linux-block@vger.kernel.org, John Garry <john.garry@huawei.com>,
	Hannes Reinecke <hare@suse.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ming Lei <ming.lei@redhat.com>
Subject: Re: [PATCH 8/8] blk-mq: drain I/O when all CPUs in a hctx are offline
Date: Wed, 27 May 2020 16:09:19 -0700	[thread overview]
Message-ID: <7acc7ab5-02f9-e6ee-e95f-175bc0df9cbc@acm.org> (raw)
In-Reply-To: <20200527180644.514302-9-hch@lst.de>

On 2020-05-27 11:06, Christoph Hellwig wrote:
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -180,6 +180,14 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
>  	sbitmap_finish_wait(bt, ws, &wait);
>  
>  found_tag:
> +	/*
> +	 * Give up this allocation if the hctx is inactive.  The caller will
> +	 * retry on an active hctx.
> +	 */
> +	if (unlikely(test_bit(BLK_MQ_S_INACTIVE, &data->hctx->state))) {
> +		blk_mq_put_tag(tags, data->ctx, tag + tag_offset);
> +		return -1;
> +	}
>  	return tag + tag_offset;
>  }

The code that has been added in blk_mq_hctx_notify_offline() will only
work correctly if blk_mq_get_tag() tests BLK_MQ_S_INACTIVE after the
store instructions involved in the tag allocation happened. Does this
mean that a memory barrier should be added in the above function before
the test_bit() call?

Thanks,

Bart.