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From:   Kalle Valo <kvalo@codeaurora.org>
To:     Wen Gong <wgong@codeaurora.org>
Cc:     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
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Date:   Wed, 09 Dec 2020 11:24:04 +0200
In-Reply-To: <010101746bd17881-819242de-7cbb-4df3-93e2-59473d281155-000000@us-west-2.amazonses.com>
        (Wen Gong's message of "Tue, 8 Sep 2020 03:45:24 +0000")
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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

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