Message-ID: <20140902142314.GX5001@linux.vnet.ibm.com>
Date:	Tue, 2 Sep 2014 07:23:14 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kgdb-bugreport@...ts.sourceforge.net, patches@...aro.org,
	linaro-kernel@...ts.linaro.org,
	John Stultz <john.stultz@...aro.org>,
	Anton Vorontsov <anton.vorontsov@...aro.org>,
	Colin Cross <ccross@...roid.com>, kernel-team@...roid.com,
	Rob Herring <robherring2@...il.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Dave Martin <Dave.Martin@....com>,
	Fabio Estevam <festevam@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Nicolas Pitre <nico@...aro.org>
Subject: Re: [PATCH v10 03/19] arm: fiq: Replace default FIQ handler

On Tue, Sep 02, 2014 at 12:49:16PM +0100, Daniel Thompson wrote:
> On 28/08/14 16:01, Russell King - ARM Linux wrote:
> > On Tue, Aug 19, 2014 at 07:12:07PM +0100, Daniel Thompson wrote:
> >> On 19/08/14 18:37, Russell King - ARM Linux wrote:
> >>> On Tue, Aug 19, 2014 at 05:45:53PM +0100, Daniel Thompson wrote:
> >>>> +int register_fiq_nmi_notifier(struct notifier_block *nb)
> >>>> +{
> >>>> +	return atomic_notifier_chain_register(&fiq_nmi_chain, nb);
> >>>> +}
> >>>> +
> >>>> +asmlinkage void __exception_irq_entry fiq_nmi_handler(struct pt_regs *regs)
> >>>> +{
> >>>> +	struct pt_regs *old_regs = set_irq_regs(regs);
> >>>> +
> >>>> +	nmi_enter();
> >>>> +	atomic_notifier_call_chain(&fiq_nmi_chain, (unsigned long)regs, NULL);
> >>>> +	nmi_exit();
> >>>> +	set_irq_regs(old_regs);
> >>>> +}
> >>>
> >>> Really not happy with this.  What happens if a FIQ occurs while we're
> >>> inside register_fiq_nmi_notifier() - more specifically inside
> >>> atomic_notifier_chain_register() ?
> >>
> >> Should depend on which side of the rcu update we're on.
> > 
> > I just asked Paul McKenney, our RCU expert... essentially, yes, RCU
> > stuff itself is safe in this context.  However, RCU stuff can call into
> > lockdep if lockdep is configured, and there are questions over lockdep.
> > 
> > There's some things which can be done to reduce the lockdep exposure
> > to it, such as ensuring that rcu_read_lock() is first called outside
> > of FIQ context.
> > 
> > There's concerns with whether either printk() in check_flags() could
> > be reached too (flags there should always indicate that IRQs were
> > disabled, so that reduces down to a question about just the first
> > printk() there.)
> > 
> > There's also the very_verbose() stuff for RCU lockdep classes which
> > Paul says must not be enabled.
> > 
> > Lastly, Paul isn't a lockdep expert, but he sees nothing that prevents
> > lockdep doing the deadlock checking as a result of the above call.
> > 
> > So... this coupled with my feeling that notifiers make it too easy for
> > unreviewed code to be hooked into this path, I'm fairly sure that we
> > don't want to be calling atomic notifier chains from FIQ context.
> 
> Having esablished (elsewhere in the thread) that RCU usage is safe
> from FIQ I have been working on the assumption that your feeling
> regarding unreviewed code is sufficient on its own to avoid using
> notifiers (and also to avoid a list of function pointers like on x86).

There was a later clarification from a lockdep expert showing that the
code was in fact safe, so the notifier approach should be just fine.

							Thanx, Paul

> Therefore I have made the changes requested and produced a
> before/after patch to show the impact of this. I will merge this
> into the FIQ patchset shortly (I need to run a few more build tests
> first).
> 
> Personally I still favour using notifiers and think the coupling below is
> excessive. Nevertheless I've run a couple of basic tests on the code
> below and it works fine.
> 
> 
> diff --git a/arch/arm/include/asm/fiq.h b/arch/arm/include/asm/fiq.h
> index 175bfed..a25c952 100644
> --- a/arch/arm/include/asm/fiq.h
> +++ b/arch/arm/include/asm/fiq.h
> @@ -54,7 +54,6 @@ extern void disable_fiq(int fiq);
>  extern int ack_fiq(int fiq);
>  extern void eoi_fiq(int fiq);
>  extern bool has_fiq(int fiq);
> -extern int register_fiq_nmi_notifier(struct notifier_block *nb);
>  extern void fiq_register_mapping(int irq, struct fiq_chip *chip);
> 
>  /* helpers defined in fiqasm.S: */
> diff --git a/arch/arm/include/asm/kgdb.h b/arch/arm/include/asm/kgdb.h
> index 6563da0..cb5ccd6 100644
> --- a/arch/arm/include/asm/kgdb.h
> +++ b/arch/arm/include/asm/kgdb.h
> @@ -51,6 +51,7 @@ extern void kgdb_handle_bus_error(void);
>  extern int kgdb_fault_expected;
> 
>  extern int kgdb_register_fiq(unsigned int fiq);
> +extern void kgdb_handle_fiq(struct pt_regs *regs);
> 
>  #endif /* !__ASSEMBLY__ */
> 
> diff --git a/arch/arm/kernel/fiq.c b/arch/arm/kernel/fiq.c
> index b2bd1c7..7422b58 100644
> --- a/arch/arm/kernel/fiq.c
> +++ b/arch/arm/kernel/fiq.c
> @@ -43,12 +43,14 @@
>  #include <linux/irq.h>
>  #include <linux/radix-tree.h>
>  #include <linux/slab.h>
> +#include <linux/irqchip/arm-gic.h>
> 
>  #include <asm/cacheflush.h>
>  #include <asm/cp15.h>
>  #include <asm/exception.h>
>  #include <asm/fiq.h>
>  #include <asm/irq.h>
> +#include <asm/kgdb.h>
>  #include <asm/traps.h>
> 
>  #define FIQ_OFFSET ({					\
> @@ -65,7 +67,6 @@ static unsigned long no_fiq_insn;
>  static int fiq_start = -1;
>  static RADIX_TREE(fiq_data_tree, GFP_KERNEL);
>  static DEFINE_MUTEX(fiq_data_mutex);
> -static ATOMIC_NOTIFIER_HEAD(fiq_nmi_chain);
> 
>  /* Default reacquire function
>   * - we always relinquish FIQ control
> @@ -218,17 +219,23 @@ bool has_fiq(int fiq)
>  }
>  EXPORT_SYMBOL(has_fiq);
> 
> -int register_fiq_nmi_notifier(struct notifier_block *nb)
> -{
> -	return atomic_notifier_chain_register(&fiq_nmi_chain, nb);
> -}
> -
>  asmlinkage void __exception_irq_entry fiq_nmi_handler(struct pt_regs *regs)
>  {
>  	struct pt_regs *old_regs = set_irq_regs(regs);
> 
>  	nmi_enter();
> -	atomic_notifier_call_chain(&fiq_nmi_chain, (unsigned long)regs, NULL);
> +
> +	/* these callbacks deliberately avoid using a notifier chain in
> +	 * order to ensure code review happens (drivers cannot "secretly"
> +	 * employ FIQ without modifying this chain of calls).
> +	 */
> +#ifdef CONFIG_KGDB_FIQ
> +	kgdb_handle_fiq(regs);
> +#endif
> +#ifdef CONFIG_ARM_GIC
> +	gic_handle_fiq_ipi();
> +#endif
> +
>  	nmi_exit();
>  	set_irq_regs(old_regs);
>  }
> diff --git a/arch/arm/kernel/kgdb.c b/arch/arm/kernel/kgdb.c
> index b77b885..630a3ef 100644
> --- a/arch/arm/kernel/kgdb.c
> +++ b/arch/arm/kernel/kgdb.c
> @@ -312,12 +312,13 @@ struct kgdb_arch arch_kgdb_ops = {
>  };
> 
>  #ifdef CONFIG_KGDB_FIQ
> -static int kgdb_handle_fiq(struct notifier_block *nb, unsigned long arg,
> -			   void *data)
> +void kgdb_handle_fiq(struct pt_regs *regs)
>  {
> -	struct pt_regs *regs = (void *) arg;
>  	int actual;
> 
> +	if (!kgdb_fiq)
> +		return;
> +
>  	if (!kgdb_nmicallback(raw_smp_processor_id(), regs))
>  		return NOTIFY_OK;
> 
> @@ -333,11 +334,6 @@ static int kgdb_handle_fiq(struct notifier_block *nb, unsigned long arg,
>  	return NOTIFY_OK;
>  }
> 
> -static struct notifier_block kgdb_fiq_notifier = {
> -	.notifier_call = kgdb_handle_fiq,
> -	.priority = 100,
> -};
> -
>  int kgdb_register_fiq(unsigned int fiq)
>  {
>  	static struct fiq_handler kgdb_fiq_desc = { .name = "kgdb", };
> @@ -357,7 +353,6 @@ int kgdb_register_fiq(unsigned int fiq)
>  	}
> 
>  	kgdb_fiq = fiq;
> -	register_fiq_nmi_notifier(&kgdb_fiq_notifier);
> 
>  	return 0;
>  }
> diff --git a/drivers/irqchip/irq-gic.c b/drivers/irqchip/irq-gic.c
> index bda5a91..8821160 100644
> --- a/drivers/irqchip/irq-gic.c
> +++ b/drivers/irqchip/irq-gic.c
> @@ -502,13 +502,17 @@ static void __init gic_init_fiq(struct gic_chip_data *gic,
>  /*
>   * Fully acknowledge (both ack and eoi) a FIQ-based IPI
>   */
> -static int gic_handle_fiq_ipi(struct notifier_block *nb, unsigned long regs,
> -			   void *data)
> +void gic_handle_fiq_ipi(void)
>  {
>  	struct gic_chip_data *gic = &gic_data[0];
> -	void __iomem *cpu_base = gic_data_cpu_base(gic);
> +	void __iomem *cpu_base;
>  	unsigned long irqstat, irqnr;
> 
> +	if (!gic || !gic->fiq_enable)
> +		return;
> +
> +	cpu_base = gic_data_cpu_base(gic);
> +
>  	if (WARN_ON(!in_nmi()))
>  		return NOTIFY_BAD;
> 
> @@ -525,13 +529,6 @@ static int gic_handle_fiq_ipi(struct notifier_block *nb, unsigned long regs,
> 
>  	return NOTIFY_OK;
>  }
> -
> -/*
> - * Notifier to ensure IPI FIQ is acknowledged correctly.
> - */
> -static struct notifier_block gic_fiq_ipi_notifier = {
> -	.notifier_call = gic_handle_fiq_ipi,
> -};
>  #else /* CONFIG_FIQ */
>  static inline void gic_set_group_irq(void __iomem *base, unsigned int hwirq,
>  				     int group) {}
> @@ -1250,10 +1247,6 @@ void __init gic_init_bases(unsigned int gic_nr, int irq_start,
>  #ifdef CONFIG_SMP
>  		set_smp_cross_call(gic_raise_softirq);
>  		register_cpu_notifier(&gic_cpu_notifier);
> -#ifdef CONFIG_FIQ
> -		if (gic_data_fiq_enable(gic))
> -			register_fiq_nmi_notifier(&gic_fiq_ipi_notifier);
> -#endif
>  #endif
>  		set_handle_irq(gic_handle_irq);
>  	}
> diff --git a/include/linux/irqchip/arm-gic.h b/include/linux/irqchip/arm-gic.h
> index 45e2d8c..52a5676 100644
> --- a/include/linux/irqchip/arm-gic.h
> +++ b/include/linux/irqchip/arm-gic.h
> @@ -101,5 +101,8 @@ static inline void __init register_routable_domain_ops
>  {
>  	gic_routable_irq_domain_ops = ops;
>  }
> +
> +void gic_handle_fiq_ipi(void);
> +
>  #endif /* __ASSEMBLY */
>  #endif
> 
> 

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