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Date:   Thu, 10 Dec 2020 10:29:35 +0800
From:   Wen Gong <wgong@codeaurora.org>
To:     Ben Greear <greearb@candelatech.com>
Cc:     Kalle Valo <kvalo@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
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On 2020-12-09 23:00, Ben Greear wrote:
> 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?
> 
maybe because which I said before:
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.

> Thanks,
> Ben