Re: [PATCH 1/2] kernfs: dont take i_lock on inode attr read

[Ian Kent <raven@themaw.net>]

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