netdev - Re: [PATCH v2 14/14] nvme-tcp: add NVMe over TCP host driver

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Message-ID: <8736rufxmg.fsf@solarflare.com>
Date:   Wed, 21 Nov 2018 15:01:11 +0300
From:   Mikhail Skorzhinskii <mskorzhinskiy@...arflare.com>
To:     Sagi Grimberg <sagi@...mberg.me>
CC:     <linux-nvme@...ts.infradead.org>, <linux-block@...r.kernel.org>,
        <netdev@...r.kernel.org>, "David S. Miller" <davem@...emloft.net>,
        "Keith Busch" <keith.busch@...el.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 14/14] nvme-tcp: add NVMe over TCP host driver

Sagi Grimberg <sagi@...mberg.me> writes:
 > +static inline void nvme_tcp_queue_request(struct nvme_tcp_request *req)
 > +{
 > +	struct nvme_tcp_queue *queue = req->queue;
 > +
 > +	spin_lock_bh(&queue->lock);
 > +	list_add_tail(&req->entry, &queue->send_list);
 > +	spin_unlock_bh(&queue->lock);
 > +
 > +	queue_work_on(queue->io_cpu, nvme_tcp_wq, &queue->io_work);
 > +}

May be I missing something, but why bother with bottom half version of
locking?

There are few places where this lock could be accessed:

 (1) From ->queue_rq() call;
 (2) From submitting new AEN request;
 (3) From receiving new R2T;

Which one if these originates from bottom half? Not 100% about queue_rq
data path, but (2) and (3) looks perfectly safe for me.

Possibly just a relic of some previous iterations of experimenting?

Mikhail Skorzhinskii