sh: Allow multiple stack unwinders to be setup

Provide an interface for registering stack unwinders, where each
unwinder is given a rating that describes its accuracy and
complexity. The more accurate an unwinder is, the more complex it is.

If a the current stack unwinder faults, then the stack unwinder with the
next highest accuracy will be used in its place (provided one is
available). For example, this allows unwinders, such as the DWARF
unwinder, to liberally sprinkle BUG()s to catch badly formed DWARF debug
info.
Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/include/asm/unwinder.h

+#ifndef _LINUX_UNWINDER_H
+#define _LINUX_UNWINDER_H
+
+#include <asm/stacktrace.h>
+
+struct unwinder {
+	const char *name;
+	struct list_head list;
+	int rating;
+	void (*dump)(struct task_struct *, struct pt_regs *,
+		     unsigned long *, const struct stacktrace_ops *, void *);
+};
+
+extern int unwinder_init(void);
+extern int unwinder_register(struct unwinder *);
+
+extern void unwind_stack(struct task_struct *, struct pt_regs *,
+			 unsigned long *, const struct stacktrace_ops *,
+			 void *);
+
+extern void stack_reader_dump(struct task_struct *, struct pt_regs *,
+			      unsigned long *, const struct stacktrace_ops *,
+			      void *);
+
+#endif /* _LINUX_UNWINDER_H */

arch/sh/kernel/Makefile_32

@@ -11,8 +11,8 @@ endif
 
 obj-y	:= debugtraps.o dumpstack.o idle.o io.o io_generic.o irq.o	\
 	   machvec.o process_32.o ptrace_32.o setup.o signal_32.o	\
-	   sys_sh.o sys_sh32.o syscalls_32.o time.o topology.o	\
-	   traps.o traps_32.o
+	   sys_sh.o sys_sh32.o syscalls_32.o time.o topology.o		\
+	   traps.o traps_32.o unwinder.o
 
 obj-y				+= cpu/
 obj-$(CONFIG_VSYSCALL)		+= vsyscall/

arch/sh/kernel/Makefile_64

@@ -2,7 +2,7 @@ extra-y	:= head_64.o init_task.o vmlinux.lds
 
 obj-y	:= debugtraps.o idle.o io.o io_generic.o irq.o machvec.o process_64.o \
 	   ptrace_64.o setup.o signal_64.o sys_sh.o sys_sh64.o \
-	   syscalls_64.o time.o topology.o traps.o traps_64.o
+	   syscalls_64.o time.o topology.o traps.o traps_64.o unwinder.o
 
 obj-y				+= cpu/
 obj-$(CONFIG_SMP)		+= smp.o

arch/sh/kernel/unwinder.c

+/*
+ * 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);
+}