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

[Nicolas Pitre <nicolas.pitre@linaro.org>]

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@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 | 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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