Date:   Sun, 29 Jan 2023 07:46:09 +0100
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Yu Kuai <yukuai1@huaweicloud.com>
Cc:     Zhong Jinghua <zhongjinghua@huawei.com>, jejb@linux.ibm.com,
        martin.petersen@oracle.com, hare@suse.de, bvanassche@acm.org,
        emilne@redhat.com, linux-kernel@vger.kernel.org,
        linux-scsi@vger.kernel.org, yi.zhang@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
Subject: Re: [PATCH-next v2 2/2] scsi: fix iscsi rescan fails to create block
 device
Message-ID: <Y9YWMQ4TuJfuXAzL@kroah.com>
References: <20230128094146.205858-1-zhongjinghua@huawei.com>
 <20230128094146.205858-3-zhongjinghua@huawei.com>
 <Y9T8uQYEaGUZwpHO@kroah.com>
 <b4927ca9-7330-3f32-f68f-1a449473a0ce@huaweicloud.com>
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On Sun, Jan 29, 2023 at 09:13:55AM +0800, Yu Kuai wrote:
> Hi, Greg
> 
> 在 2023/01/28 18:45, Greg KH 写道:
> > > diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c
> > > index cac7c902cf70..a22109cdb8ef 100644
> > > --- a/drivers/scsi/scsi_sysfs.c
> > > +++ b/drivers/scsi/scsi_sysfs.c
> > > @@ -1535,9 +1535,7 @@ static void __scsi_remove_target(struct scsi_target *starget)
> > >   		if (sdev->channel != starget->channel ||
> > >   		    sdev->id != starget->id)
> > >   			continue;
> > > -		if (sdev->sdev_state == SDEV_DEL ||
> > > -		    sdev->sdev_state == SDEV_CANCEL ||
> > > -		    !get_device(&sdev->sdev_gendev))
> > > +		if (!get_device_unless_zero(&sdev->sdev_gendev))
> > 
> > If sdev_gendev is 0 here, the object is gone and you are working with
> > memory that is already freed so something is _VERY_ wrong.
> 
> In fact, this patch will work:
> 
> In __scsi_remove_target(), 'host_lock' is held to protect iterating
> siblings, and object will wait for this lock in
> scsi_device_dev_release() to remove siblings. Hence sdev will not be
> freed untill the lock is released.

Then you got lucky, as that is not how a reference counted object should
be working (i.e. the reference dropped to 0 and it still be kept alive.)

Please fix up the scsi logic here, don't abuse the reference count code.

thanks,

greg k-h