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

[Imran Khan]

Hello Ian,
Sorry for late reply. I was about to reply this week.

On 27/7/2023 10:38 am, Ian Kent wrote:
> On 20/7/23 10:03, Ian Kent wrote:
>> On Wed, 2023-07-19 at 12:23 +0800, Ian Kent wrote:

[...]
>> I do see a problem with recent changes.
>>
>> I'll send this off to Greg after I've done some testing (primarily just
>> compile and function).
>>
>> Here's a patch which describes what I found.
>>
>> Comments are, of course, welcome, ;)
> 
> Anders I was hoping you would check if/what lockdep trace
> 
> you get with this patch.
> 
> 
> Imran, I was hoping you would comment on my change as it
> 
> relates to the kernfs_iattr_rwsem changes.
> 
> 
> Ian
> 
>>
>> kernfs: fix missing kernfs_iattr_rwsem locking
>>
>> From: Ian Kent <raven@xxxxxxxxxx>
>>
>> When the kernfs_iattr_rwsem was introduced a case was missed.
>>
>> The update of the kernfs directory node child count was also protected
>> by the kernfs_rwsem and needs to be included in the change so that the
>> child count (and so the inode n_link attribute) does not change while
>> holding the rwsem for read.
>>

kernfs direcytory node's child count changes in kernfs_(un)link_sibling and
these are getting invoked while adding (kernfs_add_one),
removing(__kernfs_remove) or moving (kernfs_rename_ns)a node. Each of these
operations proceed under kernfs_rwsem and I see each invocation of
kernfs_link/unlink_sibling during the above mentioned operations is happening
under kernfs_rwsem.
So the child count should still be protected by kernfs_rwsem and we should not
need to acquire kernfs_iattr_rwsem in kernfs_link/unlink_sibling.

Kindly let me know your thoughts. I would still like to see new lockdep traces
with this change.

Thanks,
Imran

>> Fixes: 9caf696142 (kernfs: Introduce separate rwsem to protect inode
>> attributes)
>>
>> Signed-off-by: Ian Kent <raven@xxxxxxxxxx>
>> Cc: Anders Roxell <anders.roxell@xxxxxxxxxx>
>> Cc: Imran Khan <imran.f.khan@xxxxxxxxxx>
>> Cc: Arnd Bergmann <arnd@xxxxxxxx>
>> Cc: Minchan Kim <minchan@xxxxxxxxxx>
>> Cc: Eric Sandeen <sandeen@xxxxxxxxxxx>
>> ---
>>   fs/kernfs/dir.c |    4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
>> index 45b6919903e6..6e84bb69602e 100644
>> --- a/fs/kernfs/dir.c
>> +++ b/fs/kernfs/dir.c
>> @@ -383,9 +383,11 @@ static int kernfs_link_sibling(struct kernfs_node
>> *kn)
>>       rb_insert_color(&kn->rb, &kn->parent->dir.children);
>>         /* successfully added, account subdir number */
>> +    down_write(&kernfs_root(kn)->kernfs_iattr_rwsem);
>>       if (kernfs_type(kn) == KERNFS_DIR)
>>           kn->parent->dir.subdirs++;
>>       kernfs_inc_rev(kn->parent);
>> +    up_write(&kernfs_root(kn)->kernfs_iattr_rwsem);
>>         return 0;
>>   }
>> @@ -408,9 +410,11 @@ static bool kernfs_unlink_sibling(struct
>> kernfs_node *kn)
>>       if (RB_EMPTY_NODE(&kn->rb))
>>           return false;
>>   +    down_write(&kernfs_root(kn)->kernfs_iattr_rwsem);
>>       if (kernfs_type(kn) == KERNFS_DIR)
>>           kn->parent->dir.subdirs--;
>>       kernfs_inc_rev(kn->parent);
>> +    up_write(&kernfs_root(kn)->kernfs_iattr_rwsem);
>>         rb_erase(&kn->rb, &kn->parent->dir.children);
>>       RB_CLEAR_NODE(&kn->rb);
>>