From: Ian Kent <raven@themaw.net>
To: Imran Khan <imran.f.khan@oracle.com>,
Anders Roxell <anders.roxell@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>, Tejun Heo <tj@kernel.org>,
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
Subject: Re: [PATCH 1/2] kernfs: dont take i_lock on inode attr read
Date: Fri, 28 Jul 2023 08:16:52 +0800 [thread overview]
Message-ID: <70d667af-661b-6f62-aa29-a3b8610feda6@themaw.net> (raw)
In-Reply-To: <996e11bf-5f22-3ab7-2951-92109649195d@themaw.net>
On 28/7/23 08:00, Ian Kent wrote:
> On 27/7/23 12:30, Imran Khan wrote:
>> 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@themaw.net>
>>>>
>>>> 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.
>
> Yes, that's exactly what I intended (assuming you mean write lock in
> those cases)
>
> when I did it so now I wonder what I saw that lead to my patch, I'll
> need to look
>
> again ...
Ahh, I see why I thought this ...
It's the hunk:
@@ -285,10 +285,10 @@ int kernfs_iop_permission(struct mnt_idmap *idmap,
kn = inode->i_private;
root = kernfs_root(kn);
- down_read(&root->kernfs_rwsem);
+ down_read(&root->kernfs_iattr_rwsem);
kernfs_refresh_inode(kn, inode);
ret = generic_permission(&nop_mnt_idmap, inode, mask);
- up_read(&root->kernfs_rwsem);
+ up_read(&root->kernfs_iattr_rwsem);
return ret;
}
which takes away the kernfs_rwsem and introduces the possibility of
the change. It may be more instructive to add back taking the read
lock of kernfs_rwsem in .permission() than altering the sibling link
and unlink functions, I mean I even caught myself on it.
Ian
>
>
>>
>> Kindly let me know your thoughts. I would still like to see new
>> lockdep traces
>> with this change.
>
> Indeed, I hope Anders can find time to get the trace.
>
>
> Ian
>
>>
>> Thanks,
>> Imran
>>
>>>> Fixes: 9caf696142 (kernfs: Introduce separate rwsem to protect inode
>>>> attributes)
>>>>
>>>> Signed-off-by: Ian Kent <raven@themaw.net>
>>>> Cc: Anders Roxell <anders.roxell@linaro.org>
>>>> Cc: Imran Khan <imran.f.khan@oracle.com>
>>>> Cc: Arnd Bergmann <arnd@arndb.de>
>>>> Cc: Minchan Kim <minchan@kernel.org>
>>>> Cc: Eric Sandeen <sandeen@sandeen.net>
>>>> ---
>>>> 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);
>>>>
next prev parent reply other threads:[~2023-07-28 0:17 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-18 2:32 [PATCH 0/2] kernfs: remove i_lock usage that isn't needed Ian Kent
2022-10-18 2:32 ` [PATCH 1/2] kernfs: dont take i_lock on inode attr read Ian Kent
2022-10-24 8:50 ` Miklos Szeredi
2022-10-31 22:30 ` Tejun Heo
2022-12-21 13:34 ` Anders Roxell
2022-12-22 23:11 ` Ian Kent
2022-12-29 9:20 ` Arnd Bergmann
2022-12-29 13:07 ` Ian Kent
2023-01-23 3:11 ` Ian Kent
2023-07-18 19:00 ` Anders Roxell
2023-07-19 4:23 ` Ian Kent
2023-07-20 2:03 ` Ian Kent
2023-07-26 13:49 ` Miklos Szeredi
2023-07-27 0:38 ` Ian Kent
2023-07-27 4:30 ` Imran Khan
2023-07-27 5:35 ` Imran Khan
2023-07-28 0:00 ` Ian Kent
2023-07-28 0:16 ` Ian Kent [this message]
2023-07-28 1:06 ` Imran Khan
2023-07-28 1:29 ` Ian Kent
2022-10-18 2:32 ` [PATCH 2/2] kernfs: dont take i_lock on revalidate Ian Kent
2022-10-24 8:38 ` Miklos Szeredi
2022-10-31 22:31 ` Tejun Heo
2022-11-01 7:46 ` Amir Goldstein
2022-11-01 8:09 ` Ian Kent
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