Re: [-fixes] riscv: kexec: Avoid deadlock in kexec crash path

[Alexandre Ghiti]

Hi Song, Ryo,

On 06/05/2024 07:10, takakura@xxxxxxxxxxxxx wrote:
> Hi Song and Paul!
>
> > > To avoid the deadlock, this patch directly EOI the irq regardless of
> > > the active status of irqchip.
> > Taking a quick look at the other architectures, looks like no one else is
> > doing this.  Is this addressing a RISC-V-only problem?
> >
> > > diff --git a/arch/riscv/kernel/machine_kexec.c b/arch/riscv/kernel/machine_kexec.c
> > > index f6c7135b00d7..d7ddf4d2b243 100644
> > > --- a/arch/riscv/kernel/machine_kexec.c
> > > +++ b/arch/riscv/kernel/machine_kexec.c
> > > @@ -149,20 +149,12 @@ static void machine_kexec_mask_interrupts(void)
> > >   
> > >   	for_each_irq_desc(i, desc) {
> > >   		struct irq_chip *chip;
> > > -		int ret;
> > >   
> > >   		chip = irq_desc_get_chip(desc);
> > >   		if (!chip)
> > >   			continue;
> > >   
> > > -		/*
> > > -		 * First try to remove the active state. If this
> > > -		 * fails, try to EOI the interrupt.
> > > -		 */
> > > -		ret = irq_set_irqchip_state(i, IRQCHIP_STATE_ACTIVE, false);
> > > -
> > > -		if (ret && irqd_irq_inprogress(&desc->irq_data) &&
> > > -		    chip->irq_eoi)
> > > +		if (chip->irq_eoi && irqd_irq_inprogress(&desc->irq_data))
> > >   			chip->irq_eoi(&desc->irq_data);
> I think this deadlock is relevant to riscv and arm64 as they both
> acquire irqdesc spinlock by calling irq_set_irqchip_state() during their
> machine_kexec_mask_interrupts().
>
> However, I think calling irq_set_irqchip_state() during
> machine_kexec_mask_interrupts() is arm64 specific way of handling EOI
> which is not necessary for riscv.
> For arm64, its interrupt controller(gic) seems to have two ways of EOIing
> an interrupt depending on the mode which gic is configured. One of them
> treats EOI as two step procedure, priority drop and deactivation. I think
> irq_set_irqchip_state() is there to handle the deactivation part of
> the procedure.
> For riscv, EOI only requires irq_eoi handler to complete EOI and I think
> keeping irq_set_irqchip_state() will only leave this possible deadlock
> without any use.
> So I think it's best we simply remove irq_set_irqchip_state() as Song did.


I think this ^ is relevant and should be added to the commit log. @Song 
can you respin another version with the updated commit log? @Ryo can you 
add your Reviewed-by when it's done?

This fix has been lagging behind for quite some time, it would be nice 
to merge this in 6.10 and backport to stable.

Thanks,

Alex


> Sincerely,
> Ryo Takakura
>
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