Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Feedback-ID: i31e841b0:Fastmail
Message-ID: <9e35cf66-79ef-1f13-dc6b-b013c73a9fc6@themaw.net>
Date:   Thu, 29 Dec 2022 21:07:04 +0800
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.6.0
Subject: Re: [PATCH 1/2] kernfs: dont take i_lock on inode attr read
To:     Arnd Bergmann <arnd@arndb.de>,
        Anders Roxell <anders.roxell@linaro.org>,
        Tejun Heo <tj@kernel.org>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Minchan Kim <minchan@kernel.org>,
        Eric Sandeen <sandeen@sandeen.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Rick Lindsley <ricklind@linux.vnet.ibm.com>,
        David Howells <dhowells@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Carlos Maiolino <cmaiolino@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>,
        elver@google.com
References: <166606025456.13363.3829702374064563472.stgit@donald.themaw.net>
 <166606036215.13363.1288735296954908554.stgit@donald.themaw.net>
 <Y2BMonmS0SdOn5yh@slm.duckdns.org> <20221221133428.GE69385@mutt>
 <7815c8da-7d5f-c2c5-9dfd-7a77ac37c7f7@themaw.net>
 <e25ee08c-7692-4042-9961-a499600f0a49@app.fastmail.com>
Content-Language: en-US
From:   Ian Kent <raven@themaw.net>
In-Reply-To: <e25ee08c-7692-4042-9961-a499600f0a49@app.fastmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk


On 29/12/22 17:20, Arnd Bergmann wrote:
> On Fri, Dec 23, 2022, at 00:11, Ian Kent wrote:
>> On 21/12/22 21:34, Anders Roxell wrote:
>>> On 2022-10-31 12:30, Tejun Heo wrote:
>>>> On Tue, Oct 18, 2022 at 10:32:42AM +0800, Ian Kent wrote:
>>>>> The kernfs write lock is held when the kernfs node inode attributes
>>>>> are updated. Therefore, when either kernfs_iop_getattr() or
>>>>> kernfs_iop_permission() are called the kernfs node inode attributes
>>>>> won't change.
>>>>>
>>>>> Consequently concurrent kernfs_refresh_inode() calls always copy the
>>>>> same values from the kernfs node.
>>>>>
>>>>> So there's no need to take the inode i_lock to get consistent values
>>>>> for generic_fillattr() and generic_permission(), the kernfs read lock
>>>>> is sufficient.
>>>>>
>>>>> Signed-off-by: Ian Kent <raven@themaw.net>
>>>> Acked-by: Tejun Heo <tj@kernel.org>
>>> Hi,
>>>
>>> Building an allmodconfig arm64 kernel on yesterdays next-20221220 and
>>> booting that in qemu I see the following "BUG: KCSAN: data-race in
>>> set_nlink / set_nlink".
>>
>> I'll check if I missed any places where set_link() could be
>> called where the link count could be different.
>>
>>
>> If there aren't any the question will then be can writing the
>> same value to this location in multiple concurrent threads
>> corrupt it?
> I think the race that is getting reported for set_nlink()
> is about this bit getting called simulatenously on multiple
> CPUs with only the read lock held for the inode:
>
>       /* Yes, some filesystems do change nlink from zero to one */
>       if (inode->i_nlink == 0)
>                 atomic_long_dec(&inode->i_sb->s_remove_count);
>       inode->__i_nlink = nlink;
>
> Since i_nlink and __i_nlink refer to the same memory location,
> the 'inode->i_nlink == 0' check can be true for all of them
> before the nonzero nlink value gets set, and this results in
> s_remove_count being decremented more than once.


Thanks for the comment Arnd.


I'll certainly have a close look at that.

It will be a little while though, given the season, ;).


Ian