netdev - Re: [RFC PATCH net] bonding: Work around lockdep_is

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Date:   Tue, 23 Mar 2021 19:34:32 +0200
From:   Maxim Mikityanskiy <maximmi@...dia.com>
To:     Leon Romanovsky <leon@...nel.org>
CC:     Jay Vosburgh <j.vosburgh@...il.com>,
        Veaceslav Falico <vfalico@...il.com>,
        Andy Gospodarek <andy@...yhouse.net>,
        Mahesh Bandewar <maheshb@...gle.com>,
        Nikolay Aleksandrov <nikolay@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Tariq Toukan <tariqt@...dia.com>, <netdev@...r.kernel.org>
Subject: Re: [RFC PATCH net] bonding: Work around lockdep_is_held false
 positives

On 2021-03-22 16:09, Leon Romanovsky wrote:
> On Mon, Mar 22, 2021 at 02:38:46PM +0200, Maxim Mikityanskiy wrote:
>> After lockdep gets triggered for the first time, it gets disabled, and
>> lockdep_enabled() will return false. It will affect lockdep_is_held(),
>> which will start returning true all the time. Normally, it just disables
>> checks that expect a lock to be held. However, the bonding code checks
>> that a lock is NOT held, which triggers a false positive in WARN_ON.
>>
>> This commit addresses the issue by replacing lockdep_is_held with
>> spin_is_locked, which should have the same effect, but without suffering
>> from disabling lockdep.
>>
>> Fixes: ee6377147409 ("bonding: Simplify the xmit function for modes that use xmit_hash")
>> Signed-off-by: Maxim Mikityanskiy <maximmi@...dia.com>
>> Reviewed-by: Tariq Toukan <tariqt@...dia.com>
>> ---
>> While this patch works around the issue, I would like to discuss better
>> options. Another straightforward approach is to extend lockdep API with
>> lockdep_is_not_held(), which will be basically !lockdep_is_held() when
>> lockdep is enabled, but will return true when !lockdep_enabled().
> 
> lockdep_assert_not_held() was added in this cycle to tip: locking/core
> https://yhbt.net/lore/all/161475935945.20312.2870945278690244669.tip-bot2@tip-bot2/
> https://yhbt.net/lore/all/878s779s9f.fsf@codeaurora.org/

Thanks for this suggestion - I wasn't aware that this macro was recently 
added and I could use it instead of spin_is_locked.

Still, I would like to figure out why the bonding code does this test at 
all. This lock is not taken by bond_update_slave_arr() itself, so why is 
that a problem in this code?

> Thanks
>