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Date:   Mon, 9 Dec 2019 19:31:49 +0100
From:   David Sterba <dsterba@suse.cz>
To:     Anand Jain <anandsuveer@gmail.com>
Cc:     dsterba@suse.cz, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 4/4] btrfs: sysfs, add devid/dev_state kobject and
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On Mon, Dec 09, 2019 at 10:05:39PM +0800, Anand Jain 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.
>        btrfs_free_device
>          kfree(device)

Ok, as long as this holds and sysfs file is removed before the device is
freed, the locking is not necessary.