arch/sh/kernel/unwinder.c

   1 /*
   2  * Copyright (C) 2009  Matt Fleming
   3  *
   4  * Based, in part, on kernel/time/clocksource.c.
   5  *
   6  * This file provides arbitration code for stack unwinders.
   7  *
   8  * Multiple stack unwinders can be available on a system, usually with
   9  * the most accurate unwinder being the currently active one.
  10  */
  11 #include <linux/errno.h>
  12 #include <linux/list.h>
  13 #include <linux/spinlock.h>
  14 #include <linux/module.h>
  15 #include <asm/unwinder.h>
  16 #include <asm/atomic.h>
  17
  18 /*
  19  * This is the most basic stack unwinder an architecture can
  20  * provide. For architectures without reliable frame pointers, e.g.
  21  * RISC CPUs, it can be implemented by looking through the stack for
  22  * addresses that lie within the kernel text section.
  23  *
  24  * Other CPUs, e.g. x86, can use their frame pointer register to
  25  * construct more accurate stack traces.
  26  */
  27 static struct list_head unwinder_list;
  28 static struct unwinder stack_reader = {
  29         .name = "stack-reader",
  30         .dump = stack_reader_dump,
  31         .rating = 50,
  32         .list = {
  33                 .next = &unwinder_list,
  34                 .prev = &unwinder_list,
  35         },
  36 };
  37
  38 /*
  39  * "curr_unwinder" points to the stack unwinder currently in use. This
  40  * is the unwinder with the highest rating.
  41  *
  42  * "unwinder_list" is a linked-list of all available unwinders, sorted
  43  * by rating.
  44  *
  45  * All modifications of "curr_unwinder" and "unwinder_list" must be
  46  * performed whilst holding "unwinder_lock".
  47  */
  48 static struct unwinder *curr_unwinder = &stack_reader;
  49
  50 static struct list_head unwinder_list = {
  51         .next = &stack_reader.list,
  52         .prev = &stack_reader.list,
  53 };
  54
  55 static DEFINE_SPINLOCK(unwinder_lock);
  56
  57 /**
  58  * select_unwinder - Select the best registered stack unwinder.
  59  *
  60  * Private function. Must hold unwinder_lock when called.
  61  *
  62  * Select the stack unwinder with the best rating. This is useful for
  63  * setting up curr_unwinder.
  64  */
  65 static struct unwinder *select_unwinder(void)
  66 {
  67         struct unwinder *best;
  68
  69         if (list_empty(&unwinder_list))
  70                 return NULL;
  71
  72         best = list_entry(unwinder_list.next, struct unwinder, list);
  73         if (best == curr_unwinder)
  74                 return NULL;
  75
  76         return best;
  77 }
  78
  79 /*
  80  * Enqueue the stack unwinder sorted by rating.
  81  */
  82 static int unwinder_enqueue(struct unwinder *ops)
  83 {
  84         struct list_head *tmp, *entry = &unwinder_list;
  85
  86         list_for_each(tmp, &unwinder_list) {
  87                 struct unwinder *o;
  88
  89                 o = list_entry(tmp, struct unwinder, list);
  90                 if (o == ops)
  91                         return -EBUSY;
  92                 /* Keep track of the place, where to insert */
  93                 if (o->rating >= ops->rating)
  94                         entry = tmp;
  95         }
  96         list_add(&ops->list, entry);
  97
  98         return 0;
  99 }
 100
 101 /**
 102  * unwinder_register - Used to install new stack unwinder
 103  * @u: unwinder to be registered
 104  *
 105  * Install the new stack unwinder on the unwinder list, which is sorted
 106  * by rating.
 107  *
 108  * Returns -EBUSY if registration fails, zero otherwise.
 109  */
 110 int unwinder_register(struct unwinder *u)
 111 {
 112         unsigned long flags;
 113         int ret;
 114
 115         spin_lock_irqsave(&unwinder_lock, flags);
 116         ret = unwinder_enqueue(u);
 117         if (!ret)
 118                 curr_unwinder = select_unwinder();
 119         spin_unlock_irqrestore(&unwinder_lock, flags);
 120
 121         return ret;
 122 }
 123
 124 int unwinder_faulted = 0;
 125
 126 /*
 127  * Unwind the call stack and pass information to the stacktrace_ops
 128  * functions. Also handle the case where we need to switch to a new
 129  * stack dumper because the current one faulted unexpectedly.
 130  */
 131 void unwind_stack(struct task_struct *task, struct pt_regs *regs,
 132                   unsigned long *sp, const struct stacktrace_ops *ops,
 133                   void *data)
 134 {
 135         unsigned long flags;
 136
 137         /*
 138          * The problem with unwinders with high ratings is that they are
 139          * inherently more complicated than the simple ones with lower
 140          * ratings. We are therefore more likely to fault in the
 141          * complicated ones, e.g. hitting BUG()s. If we fault in the
 142          * code for the current stack unwinder we try to downgrade to
 143          * one with a lower rating.
 144          *
 145          * Hopefully this will give us a semi-reliable stacktrace so we
 146          * can diagnose why curr_unwinder->dump() faulted.
 147          */
 148         if (unwinder_faulted) {
 149                 spin_lock_irqsave(&unwinder_lock, flags);
 150
 151                 /* Make sure no one beat us to changing the unwinder */
 152                 if (unwinder_faulted && !list_is_singular(&unwinder_list)) {
 153                         list_del(&curr_unwinder->list);
 154                         curr_unwinder = select_unwinder();
 155
 156                         unwinder_faulted = 0;
 157                 }
 158
 159                 spin_unlock_irqrestore(&unwinder_lock, flags);
 160         }
 161
 162         curr_unwinder->dump(task, regs, sp, ops, data);
 163 }
 164 EXPORT_SYMBOL_GPL(unwind_stack);