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Message-ID: <3f91aab6-0f50-38cb-6a88-b2553b96e8cb@alu.unizg.hr>
Date:   Tue, 25 Apr 2023 17:59:02 +0200
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Subject: Re: [PATCH v3 1/1] wifi: mac80211: fortify the spinlock against
 deadlock by interrupt
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To:     Leon Romanovsky <leon@kernel.org>
Cc:     Johannes Berg <johannes.berg@intel.com>,
        linux-wireless@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Johannes Berg <johannes@sipsolutions.net>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Gregory Greenman <gregory.greenman@intel.com>,
        Alexander Wetzel <alexander@wetzel-home.de>
References: <20230425093547.1131-1-mirsad.todorovac@alu.unizg.hr>
 <20230425153353.GB27649@unreal>
From:   Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
In-Reply-To: <20230425153353.GB27649@unreal>
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On 25.4.2023. 17:33, Leon Romanovsky wrote:
> On Tue, Apr 25, 2023 at 11:35:48AM +0200, Mirsad Goran Todorovac wrote:
>> In the function ieee80211_tx_dequeue() there is a particular locking
>> sequence:
>>
>> begin:
>> 	spin_lock(&local->queue_stop_reason_lock);
>> 	q_stopped = local->queue_stop_reasons[q];
>> 	spin_unlock(&local->queue_stop_reason_lock);
>>
>> However small the chance (increased by ftracetest), an asynchronous
>> interrupt can occur in between of spin_lock() and spin_unlock(),
>> and the interrupt routine will attempt to lock the same
>> &local->queue_stop_reason_lock again.
>>
>> This will cause a costly reset of the CPU and the wifi device or an
>> altogether hang in the single CPU and single core scenario.
>>
>> This is the probable trace of the deadlock:
>>
>> Apr 10 00:58:33 marvin-IdeaPad-3-15ITL6 kernel:  Possible unsafe locking scenario:
>> Apr 10 00:58:33 marvin-IdeaPad-3-15ITL6 kernel:        CPU0
>> Apr 10 00:58:33 marvin-IdeaPad-3-15ITL6 kernel:        ----
>> Apr 10 00:58:33 marvin-IdeaPad-3-15ITL6 kernel:   lock(&local->queue_stop_reason_lock);
>> Apr 10 00:58:33 marvin-IdeaPad-3-15ITL6 kernel:   <Interrupt>
>> Apr 10 00:58:33 marvin-IdeaPad-3-15ITL6 kernel:     lock(&local->queue_stop_reason_lock);
>> Apr 10 00:58:33 marvin-IdeaPad-3-15ITL6 kernel:
>>                                                   *** DEADLOCK ***
> 
> Can you please add to the commit message whole lockdep trace?
> 
> And please trim "Apr 10 00:58:33 marvin-IdeaPad-3-15ITL6 kernel: " line prefix,
> it doesn't add any value.

Sure. I will do this ASAP. I thought of it myself, but I reckoned it would
be an overkill.

Will come in PATCH v4.

Best regards,
Mirsad

-- 
Mirsad Todorovac
System engineer
Faculty of Graphic Arts | Academy of Fine Arts
University of Zagreb
Republic of Croatia, the European Union

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