Re: [RFC][PATCH 2/6] x86, nmi: create new NMI handler routines

Paul E. McKenney
2011-08-24T17:07:18Z

On Fri, Aug 19, 2011 at 04:37:42PM -0400, Don Zickus wrote:
> The NMI handlers used to rely on the notifier infrastructure. This worked
> great until we wanted to support handling multiple events better.
>
> One of the key ideas to the nmi handling is to process _all_ the handlers for
> each NMI. The reason behind this switch is because NMIs are edge triggered.
> If enough NMIs are triggered, then they could be lost because the cpu can
> only latch at most one NMI (besides the one currently being processed).
>
> In order to deal with this we have decided to process all the NMI handlers
> for each NMI. This allows the handlers to determine if they recieved an
> event or not (the ones that can not determine this will be left to fend
> for themselves on the unknown NMI list).
>
> As a result of this change it is now possible to have an extra NMI that
> was destined to be received for an already processed event. Because the
> event was processed in the previous NMI, this NMI gets dropped and becomes
> an 'unknown' NMI. This of course will cause printks that scare people.
>
> However, we prefer to have extra NMIs as opposed to losing NMIs and as such
> are have developed a basic mechanism to catch most of them. That will be
> a later patch.
>
> To accomplish this idea, I unhooked the nmi handlers from the notifier
> routines and created a new mechanism loosely based on doIRQ. The reason
> for this is the notifier routines have a couple of shortcomings. One we
> could't guarantee all future NMI handlers used NOTIFY_OK instead of
> NOTIFY_STOP. Second, we couldn't keep track of the number of events being
> handled in each routine (most only handle one, perf can handle more than one).
> Third, I wanted to eventually display which nmi handlers are registered in
> the system in /proc/interrupts to help see who is generating NMIs.
>
> The patch below just implements the new infrastructure but doesn't wire it up
> yet (that is the next patch). Its design is based on doIRQ structs and the
> atomic notifier routines. So the rcu stuff in the patch isn't entirely untested
> (as the notifier routines have soaked it) but it should be double checked in
> case I copied the code wrong.

One comment below.

Thanx, Paul

> Signed-off-by: Don Zickus <dzickus@redhat.com>
> ---
> arch/x86/include/asm/nmi.h | 19 ++++++
> arch/x86/kernel/nmi.c | 139 ++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 158 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/include/asm/nmi.h b/arch/x86/include/asm/nmi.h
> index 4886a68..6d04b28 100644
> --- a/arch/x86/include/asm/nmi.h
> +++ b/arch/x86/include/asm/nmi.h
> @@ -42,6 +42,25 @@ void arch_trigger_all_cpu_backtrace(void);
> #define NMI_LOCAL_NORMAL_PRIOR (NMI_LOCAL_BIT | NMI_NORMAL_PRIOR)
> #define NMI_LOCAL_LOW_PRIOR (NMI_LOCAL_BIT | NMI_LOW_PRIOR)
>
> +#define NMI_FLAG_FIRST 1
> +
> +enum {
> + NMI_LOCAL=0,
> + NMI_UNKNOWN,
> + NMI_EXTERNAL,
> + NMI_MAX
> +};
> +
> +#define NMI_DONE 0
> +#define NMI_HANDLED 1
> +
> +typedef int (*nmi_handler_t)(unsigned int, struct pt_regs *);
> +
> +int register_nmi_handler(unsigned int, nmi_handler_t, unsigned long,
> + const char *);
> +
> +void unregister_nmi_handler(unsigned int, const char *);
> +
> void stop_nmi(void);
> void restart_nmi(void);
>
> diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
> index 68d758a..dfc46a8 100644
> --- a/arch/x86/kernel/nmi.c
> +++ b/arch/x86/kernel/nmi.c
> @@ -13,6 +13,9 @@
> #include <linux/kprobes.h>
> #include <linux/kdebug.h>
> #include <linux/nmi.h>
> +#include <linux/delay.h>
> +#include <linux/hardirq.h>
> +#include <linux/slab.h>
>
> #if defined(CONFIG_EDAC)
> #include <linux/edac.h>
> @@ -21,6 +24,27 @@
> #include <linux/atomic.h>
> #include <asm/traps.h>
> #include <asm/mach_traps.h>
> +#include <asm/nmi.h>
> +
> +struct nmiaction {
> + struct nmiaction __rcu *next;
> + nmi_handler_t handler;
> + unsigned int flags;
> + const char *name;
> +};
> +
> +struct nmi_desc {
> + spinlock_t lock;
> + struct nmiaction __rcu *head;
> +};
> +
> +static struct nmi_desc nmi_desc[NMI_MAX] =
> +{
> + { .lock = __SPIN_LOCK_UNLOCKED(&nmi_desc[0].lock), },
> + { .lock = __SPIN_LOCK_UNLOCKED(&nmi_desc[1].lock), },
> + { .lock = __SPIN_LOCK_UNLOCKED(&nmi_desc[2].lock), },
> +
> +};
>
> static int ignore_nmis;
>
> @@ -38,6 +62,121 @@ static int __init setup_unknown_nmi_panic(char *str)
> }
> __setup("unknown_nmi_panic", setup_unknown_nmi_panic);
>
> +#define nmi_to_desc(type) (&nmi_desc[type])
> +
> +static int notrace __kprobes nmi_handle(unsigned int type, struct pt_regs *regs)
> +{
> + struct nmi_desc *desc = nmi_to_desc(type);
> + struct nmiaction *next_a, *a, **ap = &desc->head;
> + int handled=0;
> +
> + rcu_read_lock();
> + a = rcu_dereference_raw(*ap);

The reason for rcu_dereference_raw() is to prevent lockdep from choking
due to being called from an NMI handler, correct? If so, please add a
comment to this effect on this and similar uses.

> +
> + /*
> + * NMIs are edge-triggered, which means if you have enough
> + * of them concurrently, you can lose some because only one
> + * can be latched at any given time. Walk the whole list
> + * to handle those situations.
> + */
> + while (a) {
> + next_a = rcu_dereference_raw(a->next);
> +
> + handled += a->handler(type, regs);
> +
> + a = next_a;
> + }
> + rcu_read_unlock();
> +
> + /* return total number of NMI events handled */
> + return handled;
> +}
> +
> +static int __setup_nmi(unsigned int type, struct nmiaction *action)
> +{
> + struct nmi_desc *desc = nmi_to_desc(type);
> + struct nmiaction **a = &(desc->head);
> + unsigned long flags;
> +
> + spin_lock_irqsave(&desc->lock, flags);
> +
> + /*
> + * some handlers need to be executed first otherwise a fake
> + * event confuses some handlers (kdump uses this flag)
> + */
> + if (!(action->flags & NMI_FLAG_FIRST))
> + while ((*a) != NULL)
> + a = &((*a)->next);
> +
> + action->next = *a;
> + rcu_assign_pointer(*a, action);
> +
> + spin_unlock_irqrestore(&desc->lock, flags);
> + return 0;
> +}
> +
> +static struct nmiaction *__free_nmi(unsigned int type, const char *name)
> +{
> + struct nmi_desc *desc = nmi_to_desc(type);
> + struct nmiaction *n, **np = &(desc->head);
> + unsigned long flags;
> +
> + spin_lock_irqsave(&desc->lock, flags);
> +
> + while ((*np) != NULL) {
> + n = *np;
> +
> + /*
> + * the name passed in to describe the nmi handler
> + * is used as the lookup key
> + */
> + if (!strcmp(n->name, name)) {
> + WARN(in_nmi(),
> + "Trying to free NMI (%s) from NMI context!\n", n->name);
> + rcu_assign_pointer(*np, n->next);
> + break;
> + }
> + np = &(n->next);
> + }
> +
> + spin_unlock_irqrestore(&desc->lock, flags);
> + synchronize_rcu();
> + return *np;
> +}
> +
> +int register_nmi_handler(unsigned int type, nmi_handler_t handler,
> + unsigned long nmiflags, const char *devname)
> +{
> + struct nmiaction *action;
> + int retval;
> +
> + if (!handler)
> + return -EINVAL;
> +
> + action = kzalloc(sizeof(struct nmiaction), GFP_KERNEL);
> + if (!action)
> + return -ENOMEM;
> +
> + action->handler = handler;
> + action->flags = nmiflags;
> + action->name = devname;
> +
> + retval = __setup_nmi(type, action);
> +
> + if (retval)
> + kfree(action);
> +
> + return retval;
> +}
> +EXPORT_SYMBOL(register_nmi_handler);
> +
> +void unregister_nmi_handler(unsigned int type, const char *name)
> +{
> + kfree(__free_nmi(type, name));
> +}
> +
> +EXPORT_SYMBOL_GPL(unregister_nmi_handler);
> +
> static notrace __kprobes void
> pci_serr_error(unsigned char reason, struct pt_regs *regs)
> {
> --
> 1.7.6
>
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