linux-kernel - Re: [RFC PATCH 3/3] nvme: Introduce nvme_execute_passthru_rq

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Message-ID: <94c1e177-4848-c88b-ec26-3da118fd18dc@deltatee.com>
Date:   Mon, 28 Oct 2019 10:58:56 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        Sagi Grimberg <sagi@...mberg.me>,
        Keith Busch <kbusch@...nel.org>,
        Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [RFC PATCH 3/3] nvme: Introduce nvme_execute_passthru_rq_nowait()



On 2019-10-27 9:09 a.m., Christoph Hellwig wrote:
> On Fri, Oct 25, 2019 at 02:25:35PM -0600, Logan Gunthorpe wrote:
>> This function is similar to nvme_execute_passthru_rq() but does
>> not wait and will call a callback when the request is complete.
>>
>> The new function can also be called in interrupt context, so if there
>> are side effects, the request will be executed in a work queue to
>> avoid sleeping.
> 
> Why would you ever call it from interrupt context?  All the target
> submission handlers should run in process context.

Oh, I mis-understood this a bit and worded that incorrectly. The intent
is to avoid having to call nvme_passthru_end() in the completion handler
which can be in interrupt context.

>> +void nvme_execute_passthru_rq_nowait(struct request *rq, rq_end_io_fn *done)
>> +{
>> +	struct nvme_command *cmd = nvme_req(rq)->cmd;
>> +	struct nvme_ctrl *ctrl = nvme_req(rq)->ctrl;
>> +	struct nvme_ns *ns = rq->q->queuedata;
>> +	struct gendisk *disk = ns ? ns->disk : NULL;
>> +	u32 effects;
>> +
>> +	/*
>> +	 * This function may be called in interrupt context, so we cannot sleep
>> +	 * but nvme_passthru_[start|end]() may sleep so we need to execute
>> +	 * the command in a work queue.
>> +	 */
>> +	effects = nvme_command_effects(ctrl, ns, cmd->common.opcode);
>> +	if (effects) {
>> +		rq->end_io = done;
>> +		INIT_WORK(&nvme_req(rq)->work, nvme_execute_passthru_rq_work);
>> +		queue_work(nvme_wq, &nvme_req(rq)->work);
> 
> But independent of the target code - I'd much rather leave this to the
> caller.  Just call nvme_command_effects in the target code, then if
> there are not side effects use blk_execute_rq_nowait directly, else
> schedule a workqueue in the target code and call
> nvme_execute_passthru_rq from it.

Ok, that seems sensible. Except it conflicts a bit with Sagi's feedback:
presumably we need to cancel the work items during nvme_stop_ctrl() and
that's going to be rather difficult to do from the caller. Are we saying
this is unnecessary? It's not clear to me if passthru_start/end is going
to be affected by nvme_stop_ctrl() which I believe is the main concern.

Logan