Re: [PATCH 3.19-rc6 v16 2/6] irqchip: gic: Make gic_raise_softirq FIQ-safe

From: Nicolas Pitre
Date: Thu Feb 26 2015 - 15:33:54 EST


On Tue, 3 Feb 2015, Daniel Thompson wrote:

> It is currently possible for FIQ handlers to re-enter gic_raise_softirq()
> and lock up.
>
> gic_raise_softirq()
> lock(x);
> -~-> FIQ
> handle_fiq()
> gic_raise_softirq()
> lock(x); <-- Lockup
>
> arch/arm/ uses IPIs to implement arch_irq_work_raise(), thus this issue
> renders it difficult for FIQ handlers to safely defer work to less
> restrictive calling contexts.
>
> This patch fixes the problem by converting the cpu_map_migration_lock
> into a rwlock making it safe to re-enter the function.
>
> Note that having made it safe to re-enter gic_raise_softirq() we no
> longer need to mask interrupts during gic_raise_softirq() because the
> b.L migration is always performed from task context.

Very good point.

Once my concerns on patch #1 are addressed, you may add my ACK to this
one.

> Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Jason Cooper <jason@xxxxxxxxxxxxxx>
> Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> ---
> drivers/irqchip/irq-gic.c | 38 +++++++++++++++++++++++++-------------
> 1 file changed, 25 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/irqchip/irq-gic.c b/drivers/irqchip/irq-gic.c
> index a9ed64dcc84b..c172176499f6 100644
> --- a/drivers/irqchip/irq-gic.c
> +++ b/drivers/irqchip/irq-gic.c
> @@ -75,22 +75,25 @@ 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.
> + *
> + * This lock may be locked for reading from both IRQ and FIQ handlers
> + * and therefore must not be locked for writing when these are enabled.
> */
> #ifdef CONFIG_BL_SWITCHER
> -static DEFINE_RAW_SPINLOCK(cpu_map_migration_lock);
> +static DEFINE_RWLOCK(cpu_map_migration_lock);
>
> -static inline void bl_migration_lock(unsigned long *flags)
> +static inline void bl_migration_lock(void)
> {
> - raw_spin_lock_irqsave(&cpu_map_migration_lock, *flags);
> + read_lock(&cpu_map_migration_lock);
> }
>
> -static inline void bl_migration_unlock(unsigned long flags)
> +static inline void bl_migration_unlock(void)
> {
> - raw_spin_unlock_irqrestore(&cpu_map_migration_lock, flags);
> + read_unlock(&cpu_map_migration_lock);
> }
> #else
> -static inline void bl_migration_lock(unsigned long *flags) {}
> -static inline void bl_migration_unlock(unsigned long flags) {}
> +static inline void bl_migration_lock(void) {}
> +static inline void bl_migration_unlock(void) {}
> #endif
>
> /*
> @@ -640,12 +643,20 @@ static void __init gic_pm_init(struct gic_chip_data *gic)
> #endif
>
> #ifdef CONFIG_SMP
> +/*
> + * Raise the specified IPI on all cpus set in mask.
> + *
> + * This function is safe to call from all calling contexts, including
> + * FIQ handlers. It relies on bl_migration_lock() being multiply acquirable
> + * to avoid deadlocks when the function is re-entered at different
> + * exception levels.
> + */
> static void gic_raise_softirq(const struct cpumask *mask, unsigned int irq)
> {
> int cpu;
> - unsigned long flags, map = 0;
> + unsigned long map = 0;
>
> - bl_migration_lock(&flags);
> + bl_migration_lock();
>
> /* Convert our logical CPU mask into a physical one. */
> for_each_cpu(cpu, mask)
> @@ -660,7 +671,7 @@ 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);
>
> - bl_migration_unlock(flags);
> + bl_migration_unlock();
> }
> #endif
>
> @@ -708,7 +719,8 @@ int gic_get_cpu_id(unsigned int cpu)
> * Migrate all peripheral interrupts with a target matching the current CPU
> * to the interface corresponding to @new_cpu_id. The CPU interface mapping
> * is also updated. Targets to other CPU interfaces are unchanged.
> - * This must be called with IRQs locally disabled.
> + * This must be called from a task context and with IRQ and FIQ locally
> + * disabled.
> */
> void gic_migrate_target(unsigned int new_cpu_id)
> {
> @@ -739,9 +751,9 @@ void gic_migrate_target(unsigned int new_cpu_id)
> * 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);
> + write_lock(&cpu_map_migration_lock);
> gic_cpu_map[cpu] = 1 << new_cpu_id;
> - raw_spin_unlock(&cpu_map_migration_lock);
> + write_unlock(&cpu_map_migration_lock);
>
> /*
> * Find all the peripheral interrupts targetting the current
> --
> 1.9.3
>
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