linux-kernel - Re: [patch V2 3/6] genirq: Add optional hardware synchronization for shutdown

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Message-ID: <f516fe3a-39de-698b-eb15-c52d88c8f0c6@arm.com>
Date:   Mon, 1 Jul 2019 09:48:09 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Robert Hodaszi <Robert.Hodaszi@...i.com>
Subject: Re: [patch V2 3/6] genirq: Add optional hardware synchronization for
 shutdown

On 28/06/2019 12:11, Thomas Gleixner wrote:
> free_irq() ensures that no hardware interrupt handler is executing on a
> different CPU before actually releasing resources and deactivating the
> interrupt completely in a domain hierarchy.
> 
> But that does not catch the case where the interrupt is on flight at the
> hardware level but not yet serviced by the target CPU. That creates an
> interesing race condition:
> 
>    CPU 0                  CPU 1               IRQ CHIP
> 
>                                               interrupt is raised
>                                               sent to CPU1
> 			  Unable to handle
> 			  immediately
> 			  (interrupts off,
> 			   deep idle delay)
>    mask()
>    ...
>    free()
>      shutdown()
>      synchronize_irq()
>      release_resources()
>                           do_IRQ()
>                             -> resources are not available
> 
> That might be harmless and just trigger a spurious interrupt warning, but
> some interrupt chips might get into a wedged state.
> 
> Utilize the existing irq_get_irqchip_state() callback for the
> synchronization in free_irq().
> 
> synchronize_hardirq() is not using this mechanism as it might actually
> deadlock unter certain conditions, e.g. when called with interrupts
> disabled and the target CPU is the one on which the synchronization is
> invoked. synchronize_irq() uses it because that function cannot be called
> from non preemtible contexts as it might sleep.
> 
> No functional change intended and according to Marc the existing GIC
> implementations where the driver supports the callback should be able
> to cope with that core change. Famous last words.

I still stand by what I've said! I gave it a go on a small sample of my
 home zoo, and nothing caught fire (although it is not exactly easy to
trigger a situation where the additional synchronization would be useful).

> 
> Fixes: 464d12309e1b ("x86/vector: Switch IOAPIC to global reservation mode")
> Reported-by: Robert Hodaszi <Robert.Hodaszi@...i.com>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

FWIW:

Reviewed-and-tested-by: Marc Zyngier <marc.zyngier@....com>

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...