linux-kernel - Re: [PATCH] scsi: ufs: Fix proper API to send HPB pre-request

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Date:   Thu, 28 Oct 2021 10:07:52 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Christoph Hellwig <hch@...radead.org>,
        James Bottomley <jejb@...ux.ibm.com>
Cc:     daejun7.park@...sung.com, ALIM AKHTAR <alim.akhtar@...sung.com>,
        "avri.altman@....com" <avri.altman@....com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "huobean@...il.com" <huobean@...il.com>,
        Keoseong Park <keosung.park@...sung.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] scsi: ufs: Fix proper API to send HPB pre-request

On 10/28/21 8:27 AM, Christoph Hellwig wrote:
> On Thu, Oct 28, 2021 at 10:28:01AM -0400, James Bottomley wrote:
>> If the block people are happy with this, then I'm OK with it, but it
>> doesn't look like you've solved the fanout deadlock problem because
>> this new mechanism is still going to allocate a new tag.
> 
> Yes, same problem as before.

Hi Christoph,

I spent some time looking around for other examples of allocating and
inserting a request from inside block layer callbacks. I only found one
such example, namely in the NVMe core. nvme_timeout() calls
nvme_alloc_request() and blk_execute_rq_nowait(). The difference between
what the UFS HPB code is doing and what nvme_timeout() does doesn't seem
that big to me. For clarity, I don't like the UFS HPB protocol nor how
support for that protocol has been implemented. However, I don't see how
the UFS HPB implementation would complicate maintenance of the block
layer core. Am I perhaps missing something?

Thanks,

Bart.