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From:   Kalle Valo <kvalo@codeaurora.org>
To:     Ben Greear <greearb@candelatech.com>
Cc:     Wen Gong <wgong@codeaurora.org>, linux-wireless@vger.kernel.org,
        ath10k@lists.infradead.org
Subject: Re: [PATCH v3] ath10k: add flag to protect napi operation to avoid dead loop hang
References: <1598617348-2325-1-git-send-email-wgong@codeaurora.org>
        <87d02x1rqb.fsf@codeaurora.org>
        <010101746bd17881-819242de-7cbb-4df3-93e2-59473d281155-000000@us-west-2.amazonses.com>
        <87r1nzbadn.fsf@codeaurora.org>
        <8a97f5ba-8eba-d61b-4c92-bdfd08d388ee@candelatech.com>
Date:   Tue, 15 Dec 2020 09:56:20 +0200
In-Reply-To: <8a97f5ba-8eba-d61b-4c92-bdfd08d388ee@candelatech.com> (Ben
        Greear's message of "Wed, 9 Dec 2020 07:00:52 -0800")
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Ben Greear <greearb@candelatech.com> writes:

> On 12/9/20 1:24 AM, Kalle Valo wrote:
>> Wen Gong <wgong@codeaurora.org> writes:
>>
>>> On 2020-09-08 00:22, Kalle Valo wrote:
>>>
>>>> Just like with the recent firmware restart patch, isn't
>>>> ar->napi_enabled
>>>> racy? Wouldn't test_and_set_bit() and test_and_clear_bit() be safer?
>>>>
>>>> Or are we holding a lock? But then that should be documented with
>>>> lockdep_assert_held().
>>>
>>> yes, ath10k_hif_start is only called from ath10k_core_start, it has
>>> "lockdep_assert_held(&ar->conf_mutex)", and ath10k_hif_stop is only
>>> called from ath10k_core_stop, it also has
>>> "lockdep_assert_held(&ar->conf_mutex)". then it will not 2 thread both
>>> enter ath10k_hif_start/ath10k_hif_stop meanwhile.
>>
>> Ok, but every function depending on a lock being held should still call
>> lockdep_assert_held(), that way we can catch the bug if locking changes
>> later. So it's not enough that ath10k_core_stop() has
>> lockdep_assert_held(), also these napi functions should have it.
>>
>> I actually decided to switch using ATH10K_FLAG_NAPI_ENABLED with
>> set_bit() & co, simpler locking that way and no lockdep_assert_held()
>> needed anymore. Please check my changes in the pending branch, I have
>> only compile tested them:
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/kvalo/ath.git/commit/?h=pending&id=e0a466d296bd862080f7796b41349f9f586272c9
>>
>
> Why do you not need locking? You can't just check a bit is set and
> then do work and set it later without locking, two concurrent CPU
> threads can pass the first check and both get into the logic below it?

Good point, there is a race. I now fixed the patch in the pending and
documented that core_mutex needs to be held when changing the NAPI
state:

https://git.kernel.org/pub/scm/linux/kernel/git/kvalo/ath.git/commit/?h=pending&id=2fe769592ef6d4ae14260989dcbdbde4bff01cb6

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