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Subject: Re: [PATCH] scsi: ufs: Fix proper API to send HPB pre-request
To:     jejb@linux.ibm.com, daejun7.park@samsung.com,
        ALIM AKHTAR <alim.akhtar@samsung.com>,
        "avri.altman@wdc.com" <avri.altman@wdc.com>,
        "martin.petersen@oracle.com" <martin.petersen@oracle.com>,
        "huobean@gmail.com" <huobean@gmail.com>,
        Keoseong Park <keosung.park@samsung.com>
Cc:     "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>
References: <CGME20211027223619epcms2p60bbc74c9ba9757c58709a99acd0892ff@epcms2p6>
 <20211027223619epcms2p60bbc74c9ba9757c58709a99acd0892ff@epcms2p6>
 <0f9229c3c4c7859524411a47db96a3b53ac89c90.camel@linux.ibm.com>
From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <0d66b6d0-26c6-573f-e2a0-022e22c47b52@acm.org>
Date:   Thu, 28 Oct 2021 08:59:25 -0700
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On 10/28/21 7:28 AM, 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.

(+Jens, Christoph and linux-block)

Hi James,

My understanding is that the UFS HPB code makes ufshcd_queuecommand()
return SCSI_MLQUEUE_HOST_BUSY if the pool with pre-allocated requests is
exhausted. This will make the SCSI core reissue a SCSI command until
completion of another command has freed up one of the pre-allocated
requests. This is not the most efficient approach but should not trigger
a deadlock.

Thanks,

Bart.