Re: [PATCH v7 11/14] irqchip: ti-sci-inta: Add support for Interrupt Aggregator driver

[Lokesh Vutla]

On 29/04/19 6:41 PM, Marc Zyngier wrote:
> On 20/04/2019 11:09, Lokesh Vutla wrote:
>> Texas Instruments' K3 generation SoCs has an IP Interrupt Aggregator
>> which is an interrupt controller that does the following:
>> - Converts events to interrupts that can be understood by
>>   an interrupt router.
>> - Allows for multiplexing of events to interrupts.
>>
>> Configuration of the interrupt aggregator registers can only be done by
>> a system co-processor and the driver needs to send a message to this
>> co processor over TISCI protocol. This patch adds support for Interrupt
>> Aggregator irqdomain.
>>
>> Signed-off-by: Lokesh Vutla <lokeshvutla@xxxxxx>
>> ---
>> Changes since v6:
>> - Updated commit message.
>> - Arranged header files in alphabetical order
>> - Included vint_bit in struct ti_sci_inta_event_desc
>> - With the above change now the chip_data is event_desc instead of vint_desc
>> - No loops are used in atomic contexts.
>> - Fixed locking issue while freeing parent virq
>> - Fixed few other cosmetic changes.
>>
>>  MAINTAINERS                       |   1 +
>>  drivers/irqchip/Kconfig           |  11 +
>>  drivers/irqchip/Makefile          |   1 +
>>  drivers/irqchip/irq-ti-sci-inta.c | 589 ++++++++++++++++++++++++++++++
>>  4 files changed, 602 insertions(+)
>>  create mode 100644 drivers/irqchip/irq-ti-sci-inta.c
>>
> 
> [...]
> 
>> +/**
>> + * ti_sci_inta_alloc_irq() -  Allocate an irq within INTA domain
>> + * @domain:	irq_domain pointer corresponding to INTA
>> + * @hwirq:	hwirq of the input event
>> + *
>> + * Note: Allocation happens in the following manner:
>> + *	- Find a free bit available in any of the vints available in the list.
>> + *	- If not found, allocate a vint from the vint pool
>> + *	- Attach the free bit to input hwirq.
>> + * Return event_desc if all went ok else appropriate error value.
>> + */
>> +static struct ti_sci_inta_event_desc *ti_sci_inta_alloc_irq(struct irq_domain *domain,
>> +							    u32 hwirq)
>> +{
>> +	struct ti_sci_inta_irq_domain *inta = domain->host_data;
>> +	struct ti_sci_inta_vint_desc *vint_desc = NULL;
>> +	u16 free_bit;
>> +
>> +	mutex_lock(&inta->vint_mutex);
>> +	list_for_each_entry(vint_desc, &inta->vint_list, list) {
>> +		mutex_lock(&vint_desc->event_mutex);
>> +		free_bit = find_first_zero_bit(vint_desc->event_map,
>> +					       MAX_EVENTS_PER_VINT);
>> +		if (free_bit != MAX_EVENTS_PER_VINT) {
>> +			set_bit(free_bit, vint_desc->event_map);
>> +			mutex_unlock(&vint_desc->event_mutex);
>> +			mutex_unlock(&inta->vint_mutex);
>> +			goto alloc_event;
>> +		}
>> +		mutex_unlock(&vint_desc->event_mutex);
>> +	}
>> +	mutex_unlock(&inta->vint_mutex);
>> +
>> +	/* No free bits available. Allocate a new vint */
>> +	vint_desc = ti_sci_inta_alloc_parent_irq(domain);
>> +	if (IS_ERR(vint_desc))
>> +		return ERR_PTR(PTR_ERR(vint_desc));
>> +
>> +	mutex_lock(&vint_desc->event_mutex);
>> +	free_bit = find_first_zero_bit(vint_desc->event_map,
>> +				       MAX_EVENTS_PER_VINT);
>> +	set_bit(free_bit, vint_desc->event_map);
>> +	mutex_unlock(&vint_desc->event_mutex);
> 
> This code is still quite racy: you can have two parallel allocations
> failing to get a free bit in any of the already allocated vint_desc, and
> then both allocating a new vint_desc. If there was only one left, one of
> the allocation will fail despite having at least 63 free interrupts.

Good point. After thinking a bit more, I saw similar issue when two parallel
frees happens on a vint with only 2 bits allocated. First free when freeing
parent_irq might see all the bits cleared and does kfree(vint). Then second free
will crash when freeing parent irq.

Ill guard the entire allocation and freeing with vint_mutex and drop the
event_mutex altogether.

Thanks and regards,
Lokesh

> 
> 	M.
>