linux-kernel - Re: [PATCH] bsg referencing bus driver module

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Message-ID: <20180423183845.GA21609@xldev-tmpl.dev.purestorage.com>
Date:   Mon, 23 Apr 2018 12:38:48 -0600
From:   Anatoliy Glagolev <glagolig@...il.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
Cc:     linux-scsi@...r.kernel.org, linux-block@...r.kernel.org,
        axboe@...nel.dk, fujita.tomonori@....ntt.co.jp,
        martin.petersen@...cle.com, jthumshirn@...e.de, hare@...e.com,
        bblock@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bsg referencing bus driver module

Thanks, James. The idea of cutting communications with Scsi_Host at
bsg_unregister_queue(..) time and leaving bsg_class_device to
its own fate makes a lot of sense, conceptually. But there are
implementation issues that are difficult to work around.

bsg.c creates bsg_class_device and takes a reference to Scsi_Host
at bsg_register_queue(..) time. The reference is dropped at
bsg_class_device's release(..) function. If the driver implementing
Scsi_Host template is not around we crash.
We could move the reference drop from bsg_class_device's release(..)
function to bsg_unregister_queue(..). That would be a small change in
bsg.c. But bsg.c sets Scsi_Host as the parent of bsg_class_device's
device. We cannot have a device around with a dangling parent.
A device's parent cannot be changed dynamically. Not setting
the device's parent at creation may affect software relying
on bsg_class_device - Scsi_Host child-parent relations.

It looks like I am out of options. Do you have suggestions on
how to work around Scsi_Host being bsg_class_device's parent?