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Date:   Fri, 13 Dec 2019 18:02:15 +0100
From:   David Sterba <dsterba@suse.cz>
To:     dsterba@suse.cz, Anand Jain <anandsuveer@gmail.com>,
        linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 4/4] btrfs: sysfs, add devid/dev_state kobject and
 attribute
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On Fri, Dec 13, 2019 at 05:43:32PM +0100, David Sterba wrote:
> >   Looked into this further, actually we don't need any lock here
> >   the device delete thread which calls kobject_put() makes sure
> >   sysfs read is closed. So an existing sysfs read thread will have
> >   to complete before device free.
> > 
> > 
> >        CPU1                                   CPU2
> > 
> >   btrfs_rm_device
> >                                            open file
> >      btrfs_sysfs_rm_device_link
> >                                            call read, access freed device
> >        sysfs waits for the open file
> >        to close.
> 
> How exactly does sysfs wait for the device? Is it eg wait_event checking
> number of references? If the file stays open by an evil process is it
> going to block the device removal indefinitelly?

Yeah, sysfs waits until the file is closed. Eg. umount can be stalled
that way too.