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Date:   Thu, 15 Jun 2023 13:57:37 +0900
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Subject: Re: Fwd: Waking up from resume locks up on sr device
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To:     Christoph Hellwig <hch@lst.de>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Hannes Reinecke <hare@suse.de>,
        Joe Breuer <linux-kernel@jmbreuer.net>,
        Bart Van Assche <bvanassche@acm.org>,
        Bagas Sanjaya <bagasdotme@gmail.com>,
        Pavel Machek <pavel@ucw.cz>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Len Brown <len.brown@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kees Cook <keescook@chromium.org>,
        Tony Luck <tony.luck@intel.com>,
        "Guilherme G. Piccoli" <gpiccoli@igalia.com>,
        Thorsten Leemhuis <linux@leemhuis.info>,
        "James E.J. Bottomley" <jejb@linux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Phillip Potter <phil@philpotter.co.uk>,
        Linux Power Management <linux-pm@vger.kernel.org>,
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        Linux Regressions <regressions@lists.linux.dev>,
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        Dan Williams <dan.j.williams@intel.com>,
        Hannes Reinecke <hare@suse.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Martin Kepplinger <martin.kepplinger@puri.sm>,
        Kai-Heng Feng <kai.heng.feng@canonical.com>
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From:   Damien Le Moal <dlemoal@kernel.org>
Organization: Western Digital Research
In-Reply-To: <20230615044015.GA4720@lst.de>
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On 6/15/23 13:40, Christoph Hellwig wrote:
> On Thu, Jun 15, 2023 at 09:10:28AM +0900, Damien Le Moal wrote:
>>> Here's what commit e27829dc92e5 ("scsi: serialize ->rescan against 
>>> ->remove", written by Christoph Hellwig) says:
>>>
>>>     Lock the device embedded in the scsi_device to protect against
>>>     concurrent calls to ->remove.
>>>
>>> That's the commit which added the device_lock() call.
>>
>> Thanks for the information.
>>
>> +Christoph
>>
>> Why is adding the device_lock() needed ? We could just do a
>> scsi_device_get()+scsi_device_put() to serialize against remove. No ?
> 
> No.  scsi_device_get just increments a reference count, and thus
> prevents ->release from beeing called.  ->remove is not in any way
> affected by the refcount.

What ->remove cb are you talking about ? The gendev one ?
I am trying to understand why the use of device_lock() helps in any way given
that this is not used by any other functions in scsi. And given that
scsi_rescan_device() should always be called with a ref on the scsi device (and
so on the gendev as well) held, why would this function be racy with device remove ?

Note that I did find a couple of places where scsi_rescan_device() seems to not
be called with a reference to the scsi dev held, e.g.  store_rescan_field() and
store_state_field().

-- 
Damien Le Moal
Western Digital Research