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From:   Thomas Gleixner <tglx@linutronix.de>
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To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Johan Hovold <johan@kernel.org>,
        Eric Dumazet <edumazet@google.com>,
        netdev <netdev@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andy Shevchenko <andy.shevchenko@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-serial@vger.kernel.org
Subject: Re: [patch 1/1] genirq: Disable interrupts for force threaded handlers
In-Reply-To: <20210317144806.y4dogv6n2s62fpnw@linutronix.de>
References: <20210317143859.513307808@linutronix.de> <20210317144806.y4dogv6n2s62fpnw@linutronix.de>
Date:   Wed, 17 Mar 2021 17:09:55 +0100
Message-ID: <87pmzxyd64.fsf@nanos.tec.linutronix.de>
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On Wed, Mar 17 2021 at 15:48, Sebastian Andrzej Siewior wrote:
> On 2021-03-17 15:38:52 [+0100], Thomas Gleixner wrote:
>> thread(irq_A)
>>   irq_handler(A)
>>     spin_lock(&foo->lock);
>> 
>> interrupt(irq_B)
>>   irq_handler(B)
>>     spin_lock(&foo->lock);
>
> It will not because both threads will wake_up(thread). It is an issue if
> - if &foo->lock is shared between a hrtimer and threaded-IRQ
> - if &foo->lock is shared between a non-threaded and thread-IRQ
> - if &foo->lock is shared between a printk() in hardirq context and
>   thread-IRQ as I learned today.

That's the point and it's entirely clear from the above: A is thread
context and B is hard interrupt context and if the lock is shared then
it's busted. Otherwise we would not have this discussion at all.