linux-kernel - Re: [PATCH] drivers/core: Replace lockdep_set_novalidate

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Message-ID: <f79e93ef-cfe8-1373-7c36-15d046c0e3c5@I-love.SAKURA.ne.jp>
Date:   Thu, 9 Feb 2023 09:22:39 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     syzkaller <syzkaller@...glegroups.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH] drivers/core: Replace lockdep_set_novalidate_class() with
 unique class keys

On 2023/02/09 0:07, Alan Stern wrote:
> On Wed, Feb 08, 2023 at 07:30:25PM +0900, Tetsuo Handa wrote:
>> Commit 1704f47b50b5 ("lockdep: Add novalidate class for dev->mutex
>> conversion") made it impossible to find real deadlocks unless timing
>> dependent testings manage to trigger hung task like [1] and [2]. And
>> lockdep_set_novalidate_class() remained for more than one decade due to
>> a fear of false positives [3]. But not sharing mutex_init() could make
>> it possible to find real deadlocks without triggering hung task [4].
>> Thus, let's assign a unique class key on each "struct device"->mutex.
>>
>> Link: https://syzkaller.appspot.com/bug?extid=2d6ac90723742279e101 [1]
>> Link: https://syzkaller.appspot.com/bug?extid=2e39bc6569d281acbcfb [2]
>> Link: https://lkml.kernel.org/r/Y98FLlr7jkiFlV0k@rowland.harvard.edu [3]
>> Link: https://lkml.kernel.org/r/827177aa-bb64-87a9-e1af-dfe070744045@I-love.SAKURA.ne.jp [4]
>> Suggested-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
>> Co-developed-by: Alan Stern <stern@...land.harvard.edu>
>> Signed-off-by: Alan Stern <stern@...land.harvard.edu>
> 
> You must never do this!
> 
> I did not put my Signed-off-by: on the patch I sent to you.  I do not 
> want it added to that patch or to this one.  You should never put 
> somebody else's Signed-off-by: on a patch unless they tell you it's okay 
> to do so.

Did I misuse the Co-developed-by: tag? I added your Signed-off-by: tag because
https://docs.kernel.org/process/submitting-patches.html#when-to-use-acked-by-cc-and-co-developed-by
states that "every Co-developed-by: must be immediately followed by a Signed-off-by:
of the associated co-author."

I don't know whether the Co-developed-by: tag is used only when somebody else takes over
a previously proposed formal patch. I use the Co-developed-by: tag in order to state
developer's contribution when he/she suggested some plain diff but does not propose
that diff as a formal patch with description. Unless changes are proposed as a formal
patch (by somebody), bugs won't be fixed.

> 
> I'm happy to have people test this patch, but I do not want anybody 
> think that it is ready to be merged into the kernel.

People (and build/test bots) won't test changes that are not proposed as
a formal patch with Signed-off-by: tag. As far as I am aware, bot is not
testing plain diff.

I expected you to post a formal patch with your Signed-off-by: tag, but you didn't.
Therefore, I took over. Namely, define a dummy function for CONFIG_LOCKDEP=n case,
apply Hillf's suggestion, and reduce lines changed in kernel/locking/lockdep.c
in order to make the patch smaller and easier to apply the change.

> 
>> Co-developed-by: Hillf Danton <hdanton@...a.com>
>> Signed-off-by: Hillf Danton <hdanton@...a.com>
>> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
>> ---

>> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
>> index e3375bc40dad..74c0113646f1 100644
>> --- a/kernel/locking/lockdep.c
>> +++ b/kernel/locking/lockdep.c
>> @@ -857,6 +857,13 @@ static int static_obj(const void *obj)
>>  	 */
>>  	return is_module_address(addr) || is_module_percpu_address(addr);
>>  }
>> +
>> +int lockdep_static_obj(const void *obj)
>> +{
>> +	return static_obj(obj);
>> +}
>> +EXPORT_SYMBOL_GPL(lockdep_static_obj);
> 
> What's the point of adding a new function that just calls the old 
> function?  Why not simply rename the old function?

This makes the patch smaller and easier to apply the change. Of course,
I can update the patch if lockdep developers prefer rename over add.
What I worry is that lockdep developers do not permit static_obj() being
used by non-lockdep code.