linux-kernel - Re: Maybe a bug in kernel/irq/chip.c unmask_irq(), device IRQ masked unexpectedly. (re-formated the mail body, sorry)

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Message-ID: <alpine.DEB.2.21.1910141430310.2531@nanos.tec.linutronix.de>
Date:   Mon, 14 Oct 2019 14:34:44 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Yi Zheng <goodmenzy@...il.com>
cc:     linux-kernel@...r.kernel.org, Jason Cooper <jason@...edaemon.net>,
        Tony Lindgren <tony@...mide.com>, Sekhar Nori <nsekhar@...com>,
        Zheng Yi <yzheng@...hyauld.com>
Subject: Re: Maybe a bug in kernel/irq/chip.c unmask_irq(), device IRQ masked
 unexpectedly. (re-formated the mail body, sorry)

On Tue, 8 Oct 2019, Yi Zheng wrote:
>      There is some defects on IRQ processing:
> 
>      (1) At the beginning of handle_level_irq(), the IRQ-28 is masked, and ACK
>          action is executed: On my machine, it runs the 'else' branch:
> 
>             static inline void mask_ack_irq(struct irq_desc *desc)
>             {
>                 if (desc->irq_data.chip->irq_mask_ack) {
>                         desc->irq_data.chip->irq_mask_ack(&desc->irq_data);
>                         irq_state_set_masked(desc);
>                 } else {
>                         mask_irq(desc);
>                         if (desc->irq_data.chip->irq_ack)
>                                 desc->irq_data.chip->irq_ack(&desc->irq_data);
>                 }
>             }
> 
>          It is an 2-steps procedure:
>          1. mask_irq()
>          2. desc->irq_data.chip->irq_ack()
> 
>          the 2nd step, the function ptr is omap_mask_ack_irq(), which
>          _MASK_ the hardware INTC-IRQ-32 and then do the real ACK action.

Sure. Where is the problem?

>      (2) mask_irq()/unmask_irq() are not atomic actions: They check the
>          IRQD_IRQ_MASKED flag firstly, and then mask/unmask the irq by calling
>          the function ptrs which installed by irq controller drv.  Then, those 2
>          functions set/clear the IRQD_IRQ_MASKED flag.
> 
>          I think the sequence of the hw/sw action should be mirrored reversed:
>          mask_irq():
>             check IRQD_IRQ_MASKED;
>             set hardware IRQ mask register;
>             set software IRQD_IRQ_MASKED flag;
> 
>          unmask_irq():
>             check IRQD_IRQ_MASKED;
>             /* NOTE: should before the hw unmask action!! */
>             clear software IRQD_IRQ_MASKED flag;
>             clear hardware IRQ mask register;
> 
>          The current unmask_irq(), hw-mask action runs before sw-mask action,
>          which gives an very small time window. That cause an unexpected
>          iterated IRQ.

It's completely irrelevant because _ALL_ those operations run with
irq_desc->lock held. So nothing can actually observe that state.

>      Here is my the detail of my analyzing of handle_level_irq():
> 
>      (1) Let record the HW-IRQ-Controller Status and the SW-Flag IRQD_IRQ_MASKED
>          pair as following: (hw-mask, sw-mask).
> 
>      (2) In the 1st level of IRQ-28 ISR calling, in unmask_irq(), after the HW
>          unmask action, and before the sw-flag IRQD_IRQ_MASKED is cleared, there
>          is a VERY SMALL TIME WINDOW, in which, another IRQ-28 may triggered.
> 
>          In that time window, the mask status is (0, 1), which is no an valid
>          value.

Again. Irrelevant because not observable.

>       My fixup is in the attachment, which remove the unexpected time window of
>       IRQ iteration.

Please don't send attachments. See Documentation/process/submitting-patches.rst

Thanks,

	tglx