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Date:   Wed, 18 Jan 2023 10:40:57 +0100
From:   Johan Hovold <johan@kernel.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Johan Hovold <johan+linaro@kernel.org>,
        Marc Zyngier <maz@kernel.org>, x86@kernel.org,
        platform-driver-x86@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org,
        linux-kernel@vger.kernel.org, Dmitry Torokhov <dtor@chromium.org>,
        Jon Hunter <jonathanh@nvidia.com>,
        Hsin-Yi Wang <hsinyi@chromium.org>,
        Mark-PK Tsai <mark-pk.tsai@mediatek.com>
Subject: Re: [PATCH v4 09/19] irqdomain: Fix mapping-creation race
Message-ID: <Y8e+qfaC+ytx2LDv@hovoldconsulting.com>
References: <20230116135044.14998-1-johan+linaro@kernel.org>
 <20230116135044.14998-10-johan+linaro@kernel.org>
 <87sfg8kfh4.ffs@tglx>
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On Tue, Jan 17, 2023 at 10:39:51PM +0100, Thomas Gleixner wrote:
> On Mon, Jan 16 2023 at 14:50, Johan Hovold wrote:
> 
> > Parallel probing (e.g. due to asynchronous probing) of devices that share
> > interrupts can currently result in two mappings for the same hardware
> > interrupt to be created.
> 
> This lacks an explanation why this can happen.

The explanation is there (parallel probing), but I can amend it with the
concrete example from the input subsystem if that's what you're after?

Or do you mean that the above doesn't say that the current locking is
incomplete? I believe that's covered by the next paragraph:

    Make sure to hold the irq_domain_mutex when creating mappings so that
    looking for an existing mapping before creating a new one is done
    atomically.
 
> > @@ -802,6 +811,8 @@ unsigned int irq_create_fwspec_mapping(struct irq_fwspec *fwspec)
> >  	if (WARN_ON(type & ~IRQ_TYPE_SENSE_MASK))
> >  		type &= IRQ_TYPE_SENSE_MASK;
> >  
> > +	mutex_lock(&irq_domain_mutex);
> > +
> >  	/*
> >  	 * If we've already configured this interrupt,
> >  	 * don't do it again, or hell will break loose.
> > @@ -814,7 +825,7 @@ unsigned int irq_create_fwspec_mapping(struct irq_fwspec *fwspec)
> >  		 * interrupt number.
> >  		 */
> >  		if (type == IRQ_TYPE_NONE || type == irq_get_trigger_type(virq))
> > -			return virq;
> > +			goto out;
> >  
> >  		/*
> >  		 * If the trigger type has not been set yet, then set
> > @@ -823,36 +834,43 @@ unsigned int irq_create_fwspec_mapping(struct irq_fwspec *fwspec)
> >  		if (irq_get_trigger_type(virq) == IRQ_TYPE_NONE) {
> >  			irq_data = irq_get_irq_data(virq);
> >  			if (!irq_data)
> > -				return 0;
> > +				goto err;
> >  
> >  			irqd_set_trigger_type(irq_data, type);
> > -			return virq;
> > +			goto out;
> >  		}
> >  
> >  		pr_warn("type mismatch, failed to map hwirq-%lu for %s!\n",
> >  			hwirq, of_node_full_name(to_of_node(fwspec->fwnode)));
> > -		return 0;
> > +		goto err;
> >  	}
> >  
> >  	if (irq_domain_is_hierarchy(domain)) {
> > -		virq = irq_domain_alloc_irqs(domain, 1, NUMA_NO_NODE, fwspec);
> > +		virq = ___irq_domain_alloc_irqs(domain, -1, 1, NUMA_NO_NODE,
> > +						fwspec, false, NULL);
> >  		if (virq <= 0)
> > -			return 0;
> > +			goto err;
> >  	} else {
> >  		/* Create mapping */
> >  		virq = __irq_create_mapping_affinity(domain, hwirq, NULL);
> >  		if (!virq)
> > -			return virq;
> > +			goto err;
> >  	}
> >  
> >  	irq_data = irq_get_irq_data(virq);
> >  	if (WARN_ON(!irq_data))
> > -		return 0;
> > +		goto err;
> >  
> >  	/* Store trigger type */
> >  	irqd_set_trigger_type(irq_data, type);
> > +out:
> > +	mutex_unlock(&irq_domain_mutex);
> >  
> >  	return virq;
> > +err:
> > +	mutex_unlock(&irq_domain_mutex);
> > +
> > +	return 0;
> >  }
> >  EXPORT_SYMBOL_GPL(irq_create_fwspec_mapping);
> 
> You can spare that goto churn by renaming the existing function to
> irq_create_fwspec_mapping_locked() and invoked that guarded by the
> mutex, no?

That may be possible, but I'm not sure it's better. It wasn't really
obvious which of the above returns where error paths and which were
success paths before this change.

I'll try and see what it would look like.

Johan