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SubjectRe: [PATCH 3.18-rc3 v9 1/5] irqchip: gic: Finer grain locking for gic_raise_softirq
On 25/11/14 17:40, Marc Zyngier wrote:
> Hi Daniel,
>
> On 25/11/14 17:26, Daniel Thompson wrote:
>> irq_controller_lock is used for multiple purposes within the gic driver.
>> Primarily it is used to make register read-modify-write sequences atomic.
>> It is also used by gic_raise_softirq() in order that the big.LITTLE
>> migration logic can figure out when it is safe to migrate interrupts
>> between physical cores.
>>
>> The second usage of irq_controller_lock is difficult to discern when
>> reviewing the code because the migration itself takes place outside
>> the lock.
>>
>> This patch makes the second usage more explicit by splitting it out into
>> a separate lock and providing better comments.
>
> While we're at it, how about an additional patch that would make this
> lock disappear entirely when the big-little stuff is not compiled in,
> which is likely to be the case on a lot of (dare I say most?) systems?
> That will save expensive barriers that we definitely could do without.

Will do.


> It otherwise looks good to me.
>
> Thanks,
>
> M.
>
>> Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Jason Cooper <jason@lakedaemon.net>
>> Cc: Russell King <linux@arm.linux.org.uk>
>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>> ---
>> drivers/irqchip/irq-gic.c | 21 ++++++++++++++++++---
>> 1 file changed, 18 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/irqchip/irq-gic.c b/drivers/irqchip/irq-gic.c
>> index 38493ff28fa5..bb4bc20573ea 100644
>> --- a/drivers/irqchip/irq-gic.c
>> +++ b/drivers/irqchip/irq-gic.c
>> @@ -73,6 +73,12 @@ struct gic_chip_data {
>> static DEFINE_RAW_SPINLOCK(irq_controller_lock);
>>
>> /*
>> + * This lock is used by the big.LITTLE migration code to ensure no
>> + * IPIs can be pended on the old core after the map has been updated.
>> + */
>> +static DEFINE_RAW_SPINLOCK(cpu_map_migration_lock);
>> +
>> +/*
>> * The GIC mapping of CPU interfaces does not necessarily match
>> * the logical CPU numbering. Let's use a mapping as returned
>> * by the GIC itself.
>> @@ -624,7 +630,7 @@ static void gic_raise_softirq(const struct cpumask *mask, unsigned int irq)
>> int cpu;
>> unsigned long flags, map = 0;
>>
>> - raw_spin_lock_irqsave(&irq_controller_lock, flags);
>> + raw_spin_lock_irqsave(&cpu_map_migration_lock, flags);
>>
>> /* Convert our logical CPU mask into a physical one. */
>> for_each_cpu(cpu, mask)
>> @@ -639,7 +645,7 @@ static void gic_raise_softirq(const struct cpumask *mask, unsigned int irq)
>> /* this always happens on GIC0 */
>> writel_relaxed(map << 16 | irq, gic_data_dist_base(&gic_data[0]) + GIC_DIST_SOFTINT);
>>
>> - raw_spin_unlock_irqrestore(&irq_controller_lock, flags);
>> + raw_spin_unlock_irqrestore(&cpu_map_migration_lock, flags);
>> }
>> #endif
>>
>> @@ -710,8 +716,17 @@ void gic_migrate_target(unsigned int new_cpu_id)
>>
>> raw_spin_lock(&irq_controller_lock);
>>
>> - /* Update the target interface for this logical CPU */
>> + /*
>> + * Update the target interface for this logical CPU
>> + *
>> + * From the point we release the cpu_map_migration_lock any new
>> + * SGIs will be pended on the new cpu which makes the set of SGIs
>> + * pending on the old cpu static. That means we can defer the
>> + * migration until after we have released the irq_controller_lock.
>> + */
>> + raw_spin_lock(&cpu_map_migration_lock);
>> gic_cpu_map[cpu] = 1 << new_cpu_id;
>> + raw_spin_unlock(&cpu_map_migration_lock);
>>
>> /*
>> * Find all the peripheral interrupts targetting the current
>>
>
>



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