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Date:   Thu, 15 Jun 2023 07:09:47 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Damien Le Moal <dlemoal@kernel.org>
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Subject: Re: Fwd: Waking up from resume locks up on sr device
Message-ID: <20230615050947.GA5053@lst.de>
References: <07d6e2e7-a50a-8cf4-5c5d-200551bd6687@gmail.com> <02e4f87a-80e8-dc5d-0d6e-46939f2c74ac@acm.org> <b2cf6d96-a55a-d444-d22e-e9b3945ba43f@jmbreuer.net> <84f1c51c-86f9-04b3-0cd1-f685ebee7592@kernel.org> <37ed36f0-6f72-115c-85fb-62ef5ad72e76@suse.de> <b0fdf454-b2f7-c273-66f5-efe42fbc2807@kernel.org> <859f0eda-4984-4489-9851-c9f6ec454a88@rowland.harvard.edu> <41b069c7-8723-4507-3e5a-1d1878db9002@kernel.org> <20230615044015.GA4720@lst.de> <9633f1a6-2e33-4a40-3fdb-0c1c4e4cd720@kernel.org>
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On Thu, Jun 15, 2023 at 01:57:37PM +0900, Damien Le Moal wrote:
> > 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 ?

The one for the device locked.

> 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

The device model locks the device before calling ->remove.

> 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 ?

Because ->remove ould otherwise be called at the same time as ->rescan.

> 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().

You need both a valid reference and ensure ->remove is not called at the
same time.