Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Mime-Version: 1.0
Subject: RE: [PATCH] scsi: ufs: Fix proper API to send HPB pre-request
Reply-To: daejun7.park@samsung.com
Sender: Daejun Park <daejun7.park@samsung.com>
From:   Daejun Park <daejun7.park@samsung.com>
To:     "jejb@linux.ibm.com" <jejb@linux.ibm.com>,
        Bart Van Assche <bvanassche@acm.org>,
        Daejun Park <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>
In-Reply-To: <1d7c1faf6b6fa71599b5157ae95fc48ce479b722.camel@linux.ibm.com>
Message-ID: <20211028212034epcms2p82a6b0527e18ab6e3208e37a6188bf203@epcms2p8>
Date:   Fri, 29 Oct 2021 06:20:34 +0900
Content-Transfer-Encoding: 7bit
Content-Type: text/plain; charset="utf-8"
CMS-TYPE: 102P
DLP-Filter: Pass
References: <1d7c1faf6b6fa71599b5157ae95fc48ce479b722.camel@linux.ibm.com>
        <20211027223619epcms2p60bbc74c9ba9757c58709a99acd0892ff@epcms2p6>
        <0f9229c3c4c7859524411a47db96a3b53ac89c90.camel@linux.ibm.com>
        <0d66b6d0-26c6-573f-e2a0-022e22c47b52@acm.org>
        <CGME20211027223619epcms2p60bbc74c9ba9757c58709a99acd0892ff@epcms2p8>
Precedence: bulk

Hi James,

> On Thu, 2021-10-28 at 08:59 -0700, Bart Van Assche wrote:
> > 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.
>  
> I think the deadlock is triggered if the system is down to its last
> reserved request on the memory clearing device and the next entry in
> the queue for this device is one which does a fanout so we can't
> service it with the single reserved request we have left for the
> purposes of making forward progress.  Sending it back doesn't help,
> assuming this is the only memory clearing path, because retrying it
> won't help ... we have to succeed with a request on this path to move
> forward with clearing memory.
The above approach can retry several times (before the HPB timeout) but, it
gives up to allocate pre-request and it sends as just READ. So deadlock can
be avoided.

Thanks,
Daejun