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/*
* Copyright (C) 2009 Matt Fleming
*
* Based, in part, on kernel/time/clocksource.c.
*
* This file provides arbitration code for stack unwinders.
*
* Multiple stack unwinders can be available on a system, usually with
* the most accurate unwinder being the currently active one.
*/
#include <linux/errno.h>
#include <linux/list.h>
#include <linux/spinlock.h>
#include <asm/unwinder.h>
#include <asm/atomic.h>
/*
* This is the most basic stack unwinder an architecture can
* provide. For architectures without reliable frame pointers, e.g.
* RISC CPUs, it can be implemented by looking through the stack for
* addresses that lie within the kernel text section.
*
* Other CPUs, e.g. x86, can use their frame pointer register to
* construct more accurate stack traces.
*/
static struct list_head unwinder_list;
static struct unwinder stack_reader = {
.name = "stack-reader",
.dump = stack_reader_dump,
.rating = 50,
.list = {
.next = &unwinder_list,
.prev = &unwinder_list,
},
};
/*
* "curr_unwinder" points to the stack unwinder currently in use. This
* is the unwinder with the highest rating.
*
* "unwinder_list" is a linked-list of all available unwinders, sorted
* by rating.
*
* All modifications of "curr_unwinder" and "unwinder_list" must be
* performed whilst holding "unwinder_lock".
*/
static struct unwinder *curr_unwinder = &stack_reader;
static struct list_head unwinder_list = {
.next = &stack_reader.list,
.prev = &stack_reader.list,
};
static DEFINE_SPINLOCK(unwinder_lock);
static atomic_t unwinder_running = ATOMIC_INIT(0);
/**
* select_unwinder - Select the best registered stack unwinder.
*
* Private function. Must hold unwinder_lock when called.
*
* Select the stack unwinder with the best rating. This is useful for
* setting up curr_unwinder.
*/
static struct unwinder *select_unwinder(void)
{
struct unwinder *best;
if (list_empty(&unwinder_list))
return NULL;
best = list_entry(unwinder_list.next, struct unwinder, list);
if (best == curr_unwinder)
return NULL;
return best;
}
/*
* Enqueue the stack unwinder sorted by rating.
*/
static int unwinder_enqueue(struct unwinder *ops)
{
struct list_head *tmp, *entry = &unwinder_list;
list_for_each(tmp, &unwinder_list) {
struct unwinder *o;
o = list_entry(tmp, struct unwinder, list);
if (o == ops)
return -EBUSY;
/* Keep track of the place, where to insert */
if (o->rating >= ops->rating)
entry = tmp;
}
list_add(&ops->list, entry);
return 0;
}
/**
* unwinder_register - Used to install new stack unwinder
* @u: unwinder to be registered
*
* Install the new stack unwinder on the unwinder list, which is sorted
* by rating.
*
* Returns -EBUSY if registration fails, zero otherwise.
*/
int unwinder_register(struct unwinder *u)
{
unsigned long flags;
int ret;
spin_lock_irqsave(&unwinder_lock, flags);
ret = unwinder_enqueue(u);
if (!ret)
curr_unwinder = select_unwinder();
spin_unlock_irqrestore(&unwinder_lock, flags);
return ret;
}
/*
* Unwind the call stack and pass information to the stacktrace_ops
* functions. Also handle the case where we need to switch to a new
* stack dumper because the current one faulted unexpectedly.
*/
void unwind_stack(struct task_struct *task, struct pt_regs *regs,
unsigned long *sp, const struct stacktrace_ops *ops,
void *data)
{
unsigned long flags;
/*
* The problem with unwinders with high ratings is that they are
* inherently more complicated than the simple ones with lower
* ratings. We are therefore more likely to fault in the
* complicated ones, e.g. hitting BUG()s. If we fault in the
* code for the current stack unwinder we try to downgrade to
* one with a lower rating.
*
* Hopefully this will give us a semi-reliable stacktrace so we
* can diagnose why curr_unwinder->dump() faulted.
*/
if (atomic_inc_return(&unwinder_running) != 1) {
spin_lock_irqsave(&unwinder_lock, flags);
if (!list_is_singular(&unwinder_list)) {
list_del(&curr_unwinder->list);
curr_unwinder = select_unwinder();
}
spin_unlock_irqrestore(&unwinder_lock, flags);
atomic_dec(&unwinder_running);
}
curr_unwinder->dump(task, regs, sp, ops, data);
atomic_dec(&unwinder_running);
}
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