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Subject: Re: [PATCH 4/5] ath10k: detect conf_mutex held ath10k_drain_tx()
 calls
To:     Kalle Valo <kvalo@codeaurora.org>
Cc:     netdev@vger.kernel.org, linux-wireless@vger.kernel.org,
        linux-kernel@vger.kernel.org, ath10k@lists.infradead.org,
        kuba@kernel.org, davem@davemloft.net,
        Shuah Khan <skhan@linuxfoundation.org>
References: <cover.1612915444.git.skhan@linuxfoundation.org>
 <a980abfb143f5240375f3f1046f0f26971c695e6.1612915444.git.skhan@linuxfoundation.org>
 <87lfbwtjls.fsf@codeaurora.org>
 <d6d8c7b8-f69d-01ef-6d66-8a33ea98920f@linuxfoundation.org>
 <871rdmu9z9.fsf@codeaurora.org>
From:   Shuah Khan <skhan@linuxfoundation.org>
Message-ID: <f0905d6c-065f-a29c-2c83-77f55e77d36e@linuxfoundation.org>
Date:   Thu, 11 Feb 2021 13:53:50 -0700
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On 2/11/21 4:20 AM, Kalle Valo wrote:
> Shuah Khan <skhan@linuxfoundation.org> writes:
> 
>> On 2/10/21 1:25 AM, Kalle Valo wrote:
>>> Shuah Khan <skhan@linuxfoundation.org> writes:
>>>
>>>> ath10k_drain_tx() must not be called with conf_mutex held as workers can
>>>> use that also. Add check to detect conf_mutex held calls.
>>>>
>>>> Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
>>>
>>> The commit log does not answer to "Why?". How did you find this? What
>>> actual problem are you trying to solve?
>>>
>>
>> I came across the comment block above the ath10k_drain_tx() as I was
>> reviewing at conf_mutex holds while I was debugging the conf_mutex
>> lock assert in ath10k_debug_fw_stats_request().
>>
>> My reasoning is that having this will help detect incorrect usages
>> of ath10k_drain_tx() while holding conf_mutex which could lead to
>> locking problems when async worker routines try to call this routine.
> 
> Ok, makes sense. I prefer having this background info in the commit log,
> for example "found by code review" or something like that. Or just copy
> what you wrote above :)
> 

Thanks. I will do that.

>>>> --- a/drivers/net/wireless/ath/ath10k/mac.c
>>>> +++ b/drivers/net/wireless/ath/ath10k/mac.c
>>>> @@ -4566,6 +4566,7 @@ static void
>>>> ath10k_mac_op_wake_tx_queue(struct ieee80211_hw *hw,
>>>>    /* Must not be called with conf_mutex held as workers can use that also. */
>>>>    void ath10k_drain_tx(struct ath10k *ar)
>>>>    {
>>>> +	WARN_ON(lockdep_is_held(&ar->conf_mutex));
>>>
>>> Empty line after WARN_ON().
>>>
>>
>> Will do.
>>
>>> Shouldn't this check debug_locks similarly lockdep_assert_held() does?
>>>
>>> #define lockdep_assert_held(l)	do {				\
>>> 		WARN_ON(debug_locks && !lockdep_is_held(l));	\
>>> 	} while (0)
>>>
>>> And I suspect you need #ifdef CONFIG_LOCKDEP which should fix the kbuild
>>> bot error.
>>>
>>
>> Yes.
>>
>>> But honestly I would prefer to have lockdep_assert_not_held() in
>>> include/linux/lockdep.h, much cleaner that way. Also
>>> i915_gem_object_lookup_rcu() could then use the same macro.
>>>
>>
>> Right. This is the right way to go. That was first instinct and
>> decided to have the discussion evolve in that direction. Now that
>> it has, I will combine this change with
>> include/linux/lockdep.h and add lockdep_assert_not_held()
>>
>> I think we might have other places in the kernel that could use
>> lockdep_assert_not_held() in addition to i915_gem_object_lookup_rcu()
> 

I looked at i915_gem_object_lookup_rcu(). The following can be replaced
by lockdep_assert_held().

#ifdef CONFIG_LOCKDEP
         WARN_ON(debug_locks && !lock_is_held(&rcu_lock_map));
#endif

> Great, thank you. The only problem is that lockdep.h changes have to go
> via some other tree, I just don't know which :) I think it would be
> easiest if also the ath10k patch goes via that other tree, I can ack the
> ath10k changes.
> 
> Another option is that I'll apply the ath10k patch after the lockdep.h
> change has trickled down to my tree, but that usually happens only after
> the merge window and means weeks of waiting. Either is fine for me.
> 

I will send the include/linux/lockdep.h and ath10k patch together and
we will take it from there.

thanks,
-- Shuah