Merge branch 'sh/dwarf-unwinder'

Conflicts:
	arch/sh/kernel/cpu/sh3/entry.S

6 files changed, 213 insertions, 112 deletions

diff --git a/arch/sh/kernel/cpu/sh3/entry.S b/arch/sh/kernel/cpu/sh3/entry.S
index 272636e..8c19e21 100644
--- a/arch/sh/kernel/cpu/sh3/entry.S
+++ b/arch/sh/kernel/cpu/sh3/entry.S
@@ -508,6 +508,8 @@ ENTRY(handle_interrupt)
 	bsr	save_regs	! needs original pr value in k3
 	 mov	#-1, k2		! default vector kept in k2
 
+	setup_frame_reg
+
 	stc	sr, r0	! get status register
 	shlr2	r0
 	and	#0x3c, r0
diff --git a/arch/sh/kernel/dwarf.c b/arch/sh/kernel/dwarf.c
index 5fd6e60..577302f 100644
--- a/arch/sh/kernel/dwarf.c
+++ b/arch/sh/kernel/dwarf.c
@@ -11,12 +11,14 @@
  *
  * TODO:
  *	- DWARF64 doesn't work.
+ *	- Registers with DWARF_VAL_OFFSET rules aren't handled properly.
  */
 
 /* #define DEBUG */
 #include <linux/kernel.h>
 #include <linux/io.h>
 #include <linux/list.h>
+#include <linux/mempool.h>
 #include <linux/mm.h>
 #include <asm/dwarf.h>
 #include <asm/unwinder.h>
@@ -25,55 +27,89 @@
 #include <asm/dwarf.h>
 #include <asm/stacktrace.h>
 
+/* Reserve enough memory for two stack frames */
+#define DWARF_FRAME_MIN_REQ	2
+/* ... with 4 registers per frame. */
+#define DWARF_REG_MIN_REQ	(DWARF_FRAME_MIN_REQ * 4)
+
+static struct kmem_cache *dwarf_frame_cachep;
+static mempool_t *dwarf_frame_pool;
+
+static struct kmem_cache *dwarf_reg_cachep;
+static mempool_t *dwarf_reg_pool;
+
 static LIST_HEAD(dwarf_cie_list);
-DEFINE_SPINLOCK(dwarf_cie_lock);
+static DEFINE_SPINLOCK(dwarf_cie_lock);
 
 static LIST_HEAD(dwarf_fde_list);
-DEFINE_SPINLOCK(dwarf_fde_lock);
+static DEFINE_SPINLOCK(dwarf_fde_lock);
 
 static struct dwarf_cie *cached_cie;
 
-/*
- * Figure out whether we need to allocate some dwarf registers. If dwarf
- * registers have already been allocated then we may need to realloc
- * them. "reg" is a register number that we need to be able to access
- * after this call.
+/**
+ *	dwarf_frame_alloc_reg - allocate memory for a DWARF register
+ *	@frame: the DWARF frame whose list of registers we insert on
+ *	@reg_num: the register number
+ *
+ *	Allocate space for, and initialise, a dwarf reg from
+ *	dwarf_reg_pool and insert it onto the (unsorted) linked-list of
+ *	dwarf registers for @frame.
  *
- * Register numbers start at zero, therefore we need to allocate space
- * for "reg" + 1 registers.
+ *	Return the initialised DWARF reg.
  */
-static void dwarf_frame_alloc_regs(struct dwarf_frame *frame,
-				   unsigned int reg)
+static struct dwarf_reg *dwarf_frame_alloc_reg(struct dwarf_frame *frame,
+					       unsigned int reg_num)
 {
-	struct dwarf_reg *regs;
-	unsigned int num_regs = reg + 1;
-	size_t new_size;
-	size_t old_size;
-
-	new_size = num_regs * sizeof(*regs);
-	old_size = frame->num_regs * sizeof(*regs);
+	struct dwarf_reg *reg;
 
-	/* Fast path: don't allocate any regs if we've already got enough. */
-	if (frame->num_regs >= num_regs)
-		return;
-
-	regs = kzalloc(new_size, GFP_ATOMIC);
-	if (!regs) {
-		printk(KERN_WARNING "Unable to allocate DWARF registers\n");
+	reg = mempool_alloc(dwarf_reg_pool, GFP_ATOMIC);
+	if (!reg) {
+		printk(KERN_WARNING "Unable to allocate a DWARF register\n");
 		/*
 		 * Let's just bomb hard here, we have no way to
 		 * gracefully recover.
 		 */
-		BUG();
+		UNWINDER_BUG();
 	}
 
-	if (frame->regs) {
-		memcpy(regs, frame->regs, old_size);
-		kfree(frame->regs);
+	reg->number = reg_num;
+	reg->addr = 0;
+	reg->flags = 0;
+
+	list_add(&reg->link, &frame->reg_list);
+
+	return reg;
+}
+
+static void dwarf_frame_free_regs(struct dwarf_frame *frame)
+{
+	struct dwarf_reg *reg, *n;
+
+	list_for_each_entry_safe(reg, n, &frame->reg_list, link) {
+		list_del(&reg->link);
+		mempool_free(reg, dwarf_reg_pool);
 	}
+}
 
-	frame->regs = regs;
-	frame->num_regs = num_regs;
+/**
+ *	dwarf_frame_reg - return a DWARF register
+ *	@frame: the DWARF frame to search in for @reg_num
+ *	@reg_num: the register number to search for
+ *
+ *	Lookup and return the dwarf reg @reg_num for this frame. Return
+ *	NULL if @reg_num is an register invalid number.
+ */
+static struct dwarf_reg *dwarf_frame_reg(struct dwarf_frame *frame,
+					 unsigned int reg_num)
+{
+	struct dwarf_reg *reg;
+
+	list_for_each_entry(reg, &frame->reg_list, link) {
+		if (reg->number == reg_num)
+			return reg;
+	}
+
+	return NULL;
 }
 
 /**
@@ -196,7 +232,7 @@ static int dwarf_read_encoded_value(char *addr, unsigned long *val,
 		break;
 	default:
 		pr_debug("encoding=0x%x\n", (encoding & 0x70));
-		BUG();
+		UNWINDER_BUG();
 	}
 
 	if ((encoding & 0x07) == 0x00)
@@ -211,7 +247,7 @@ static int dwarf_read_encoded_value(char *addr, unsigned long *val,
 		break;
 	default:
 		pr_debug("encoding=0x%x\n", encoding);
-		BUG();
+		UNWINDER_BUG();
 	}
 
 	return count;
@@ -264,7 +300,7 @@ static inline int dwarf_entry_len(char *addr, unsigned long *len)
  */
 static struct dwarf_cie *dwarf_lookup_cie(unsigned long cie_ptr)
 {
-	struct dwarf_cie *cie, *n;
+	struct dwarf_cie *cie;
 	unsigned long flags;
 
 	spin_lock_irqsave(&dwarf_cie_lock, flags);
@@ -278,7 +314,7 @@ static struct dwarf_cie *dwarf_lookup_cie(unsigned long cie_ptr)
 		goto out;
 	}
 
-	list_for_each_entry_safe(cie, n, &dwarf_cie_list, link) {
+	list_for_each_entry(cie, &dwarf_cie_list, link) {
 		if (cie->cie_pointer == cie_ptr) {
 			cached_cie = cie;
 			break;
@@ -299,11 +335,12 @@ out:
  */
 struct dwarf_fde *dwarf_lookup_fde(unsigned long pc)
 {
+	struct dwarf_fde *fde;
 	unsigned long flags;
-	struct dwarf_fde *fde, *n;
 
 	spin_lock_irqsave(&dwarf_fde_lock, flags);
-	list_for_each_entry_safe(fde, n, &dwarf_fde_list, link) {
+
+	list_for_each_entry(fde, &dwarf_fde_list, link) {
 		unsigned long start, end;
 
 		start = fde->initial_location;
@@ -346,6 +383,7 @@ static int dwarf_cfa_execute_insns(unsigned char *insn_start,
 	unsigned char insn;
 	unsigned char *current_insn;
 	unsigned int count, delta, reg, expr_len, offset;
+	struct dwarf_reg *regp;
 
 	current_insn = insn_start;
 
@@ -368,9 +406,9 @@ static int dwarf_cfa_execute_insns(unsigned char *insn_start,
 			count = dwarf_read_uleb128(current_insn, &offset);
 			current_insn += count;
 			offset *= cie->data_alignment_factor;
-			dwarf_frame_alloc_regs(frame, reg);
-			frame->regs[reg].addr = offset;
-			frame->regs[reg].flags |= DWARF_REG_OFFSET;
+			regp = dwarf_frame_alloc_reg(frame, reg);
+			regp->addr = offset;
+			regp->flags |= DWARF_REG_OFFSET;
 			continue;
 			/* NOTREACHED */
 		case DW_CFA_restore:
@@ -414,6 +452,8 @@ static int dwarf_cfa_execute_insns(unsigned char *insn_start,
 		case DW_CFA_undefined:
 			count = dwarf_read_uleb128(current_insn, &reg);
 			current_insn += count;
+			regp = dwarf_frame_alloc_reg(frame, reg);
+			regp->flags |= DWARF_UNDEFINED;
 			break;
 		case DW_CFA_def_cfa:
 			count = dwarf_read_uleb128(current_insn,
@@ -452,17 +492,18 @@ static int dwarf_cfa_execute_insns(unsigned char *insn_start,
 			count = dwarf_read_leb128(current_insn, &offset);
 			current_insn += count;
 			offset *= cie->data_alignment_factor;
-			dwarf_frame_alloc_regs(frame, reg);
-			frame->regs[reg].flags |= DWARF_REG_OFFSET;
-			frame->regs[reg].addr = offset;
+			regp = dwarf_frame_alloc_reg(frame, reg);
+			regp->flags |= DWARF_REG_OFFSET;
+			regp->addr = offset;
 			break;
 		case DW_CFA_val_offset:
 			count = dwarf_read_uleb128(current_insn, &reg);
 			current_insn += count;
 			count = dwarf_read_leb128(current_insn, &offset);
 			offset *= cie->data_alignment_factor;
-			frame->regs[reg].flags |= DWARF_REG_OFFSET;
-			frame->regs[reg].addr = offset;
+			regp = dwarf_frame_alloc_reg(frame, reg);
+			regp->flags |= DWARF_VAL_OFFSET;
+			regp->addr = offset;
 			break;
 		case DW_CFA_GNU_args_size:
 			count = dwarf_read_uleb128(current_insn, &offset);
@@ -473,12 +514,14 @@ static int dwarf_cfa_execute_insns(unsigned char *insn_start,
 			current_insn += count;
 			count = dwarf_read_uleb128(current_insn, &offset);
 			offset *= cie->data_alignment_factor;
-			dwarf_frame_alloc_regs(frame, reg);
-			frame->regs[reg].flags |= DWARF_REG_OFFSET;
-			frame->regs[reg].addr = -offset;
+
+			regp = dwarf_frame_alloc_reg(frame, reg);
+			regp->flags |= DWARF_REG_OFFSET;
+			regp->addr = -offset;
 			break;
 		default:
 			pr_debug("unhandled DWARF instruction 0x%x\n", insn);
+			UNWINDER_BUG();
 			break;
 		}
 	}
@@ -495,14 +538,14 @@ static int dwarf_cfa_execute_insns(unsigned char *insn_start,
  *	on the callstack. Each of the lower (older) stack frames are
  *	linked via the "prev" member.
  */
-struct dwarf_frame *dwarf_unwind_stack(unsigned long pc,
-				       struct dwarf_frame *prev)
+struct dwarf_frame * dwarf_unwind_stack(unsigned long pc,
+					struct dwarf_frame *prev)
 {
 	struct dwarf_frame *frame;
 	struct dwarf_cie *cie;
 	struct dwarf_fde *fde;
+	struct dwarf_reg *reg;
 	unsigned long addr;
-	int i, offset;
 
 	/*
 	 * If this is the first invocation of this recursive function we
@@ -515,11 +558,16 @@ struct dwarf_frame *dwarf_unwind_stack(unsigned long pc,
 	if (!pc && !prev)
 		pc = (unsigned long)current_text_addr();
 
-	frame = kzalloc(sizeof(*frame), GFP_ATOMIC);
-	if (!frame)
-		return NULL;
+	frame = mempool_alloc(dwarf_frame_pool, GFP_ATOMIC);
+	if (!frame) {
+		printk(KERN_ERR "Unable to allocate a dwarf frame\n");
+		UNWINDER_BUG();
+	}
 
+	INIT_LIST_HEAD(&frame->reg_list);
+	frame->flags = 0;
 	frame->prev = prev;
+	frame->return_addr = 0;
 
 	fde = dwarf_lookup_fde(pc);
 	if (!fde) {
@@ -539,7 +587,7 @@ struct dwarf_frame *dwarf_unwind_stack(unsigned long pc,
 		 *	case above, which sucks because we could print a
 		 *	warning here.
 		 */
-		return NULL;
+		goto bail;
 	}
 
 	cie = dwarf_lookup_cie(fde->cie_pointer);
@@ -559,10 +607,11 @@ struct dwarf_frame *dwarf_unwind_stack(unsigned long pc,
 	switch (frame->flags) {
 	case DWARF_FRAME_CFA_REG_OFFSET:
 		if (prev) {
-			BUG_ON(!prev->regs[frame->cfa_register].flags);
+			reg = dwarf_frame_reg(prev, frame->cfa_register);
+			UNWINDER_BUG_ON(!reg);
+			UNWINDER_BUG_ON(reg->flags != DWARF_REG_OFFSET);
 
-			addr = prev->cfa;
-			addr += prev->regs[frame->cfa_register].addr;
+			addr = prev->cfa + reg->addr;
 			frame->cfa = __raw_readl(addr);
 
 		} else {
@@ -579,27 +628,30 @@ struct dwarf_frame *dwarf_unwind_stack(unsigned long pc,
 		frame->cfa += frame->cfa_offset;
 		break;
 	default:
-		BUG();
+		UNWINDER_BUG();
 	}
 
-	/* If we haven't seen the return address reg, we're screwed. */
-	BUG_ON(!frame->regs[DWARF_ARCH_RA_REG].flags);
+	reg = dwarf_frame_reg(frame, DWARF_ARCH_RA_REG);
 
-	for (i = 0; i <= frame->num_regs; i++) {
-		struct dwarf_reg *reg = &frame->regs[i];
+	/*
+	 * If we haven't seen the return address register or the return
+	 * address column is undefined then we must assume that this is
+	 * the end of the callstack.
+	 */
+	if (!reg || reg->flags == DWARF_UNDEFINED)
+		goto bail;
 
-		if (!reg->flags)
-			continue;
+	UNWINDER_BUG_ON(reg->flags != DWARF_REG_OFFSET);
 
-		offset = reg->addr;
-		offset += frame->cfa;
-	}
-
-	addr = frame->cfa + frame->regs[DWARF_ARCH_RA_REG].addr;
+	addr = frame->cfa + reg->addr;
 	frame->return_addr = __raw_readl(addr);
 
-	frame->next = dwarf_unwind_stack(frame->return_addr, frame);
 	return frame;
+
+bail:
+	dwarf_frame_free_regs(frame);
+	mempool_free(frame, dwarf_frame_pool);
+	return NULL;
 }
 
 static int dwarf_parse_cie(void *entry, void *p, unsigned long len,
@@ -624,7 +676,7 @@ static int dwarf_parse_cie(void *entry, void *p, unsigned long len,
 	cie->cie_pointer = (unsigned long)entry;
 
 	cie->version = *(char *)p++;
-	BUG_ON(cie->version != 1);
+	UNWINDER_BUG_ON(cie->version != 1);
 
 	cie->augmentation = p;
 	p += strlen(cie->augmentation) + 1;
@@ -654,7 +706,7 @@ static int dwarf_parse_cie(void *entry, void *p, unsigned long len,
 		count = dwarf_read_uleb128(p, &length);
 		p += count;
 
-		BUG_ON((unsigned char *)p > end);
+		UNWINDER_BUG_ON((unsigned char *)p > end);
 
 		cie->initial_instructions = p + length;
 		cie->augmentation++;
@@ -682,16 +734,16 @@ static int dwarf_parse_cie(void *entry, void *p, unsigned long len,
 			 * routine in the CIE
 			 * augmentation.
 			 */
-			BUG();
+			UNWINDER_BUG();
 		} else if (*cie->augmentation == 'S') {
-			BUG();
+			UNWINDER_BUG();
 		} else {
 			/*
 			 * Unknown augmentation. Assume
 			 * 'z' augmentation.
 			 */
 			p = cie->initial_instructions;
-			BUG_ON(!p);
+			UNWINDER_BUG_ON(!p);
 			break;
 		}
 	}
@@ -708,7 +760,8 @@ static int dwarf_parse_cie(void *entry, void *p, unsigned long len,
 }
 
 static int dwarf_parse_fde(void *entry, u32 entry_type,
-			   void *start, unsigned long len)
+			   void *start, unsigned long len,
+			   unsigned char *end)
 {
 	struct dwarf_fde *fde;
 	struct dwarf_cie *cie;
@@ -755,7 +808,7 @@ static int dwarf_parse_fde(void *entry, u32 entry_type,
 
 	/* Call frame instructions. */
 	fde->instructions = p;
-	fde->end = start + len;
+	fde->end = end;
 
 	/* Add to list. */
 	spin_lock_irqsave(&dwarf_fde_lock, flags);
@@ -765,17 +818,33 @@ static int dwarf_parse_fde(void *entry, u32 entry_type,
 	return 0;
 }
 
-static void dwarf_unwinder_dump(struct task_struct *task, struct pt_regs *regs,
+static void dwarf_unwinder_dump(struct task_struct *task,
+				struct pt_regs *regs,
 				unsigned long *sp,
-				const struct stacktrace_ops *ops, void *data)
+				const struct stacktrace_ops *ops,
+				void *data)
 {
-	struct dwarf_frame *frame;
+	struct dwarf_frame *frame, *_frame;
+	unsigned long return_addr;
+
+	_frame = NULL;
+	return_addr = 0;
 
-	frame = dwarf_unwind_stack(0, NULL);
+	while (1) {
+		frame = dwarf_unwind_stack(return_addr, _frame);
 
-	while (frame && frame->return_addr) {
-		ops->address(data, frame->return_addr, 1);
-		frame = frame->next;
+		if (_frame) {
+			dwarf_frame_free_regs(_frame);
+			mempool_free(_frame, dwarf_frame_pool);
+		}
+
+		_frame = frame;
+
+		if (!frame || !frame->return_addr)
+			break;
+
+		return_addr = frame->return_addr;
+		ops->address(data, return_addr, 1);
 	}
 }
 
@@ -787,24 +856,22 @@ static struct unwinder dwarf_unwinder = {
 
 static void dwarf_unwinder_cleanup(void)
 {
-	struct dwarf_cie *cie, *m;
-	struct dwarf_fde *fde, *n;
-	unsigned long flags;
+	struct dwarf_cie *cie;
+	struct dwarf_fde *fde;
 
 	/*
 	 * Deallocate all the memory allocated for the DWARF unwinder.
 	 * Traverse all the FDE/CIE lists and remove and free all the
 	 * memory associated with those data structures.
 	 */
-	spin_lock_irqsave(&dwarf_cie_lock, flags);
-	list_for_each_entry_safe(cie, m, &dwarf_cie_list, link)
+	list_for_each_entry(cie, &dwarf_cie_list, link)
 		kfree(cie);
-	spin_unlock_irqrestore(&dwarf_cie_lock, flags);
 
-	spin_lock_irqsave(&dwarf_fde_lock, flags);
-	list_for_each_entry_safe(fde, n, &dwarf_fde_list, link)
+	list_for_each_entry(fde, &dwarf_fde_list, link)
 		kfree(fde);
-	spin_unlock_irqrestore(&dwarf_fde_lock, flags);
+
+	kmem_cache_destroy(dwarf_reg_cachep);
+	kmem_cache_destroy(dwarf_frame_cachep);
 }
 
 /**
@@ -816,7 +883,7 @@ static void dwarf_unwinder_cleanup(void)
  *	easy to lookup the FDE for a given PC, so we build a list of FDE
  *	and CIE entries that make it easier.
  */
-void dwarf_unwinder_init(void)
+static int __init dwarf_unwinder_init(void)
 {
 	u32 entry_type;
 	void *p, *entry;
@@ -831,6 +898,21 @@ void dwarf_unwinder_init(void)
 	f_entries = 0;
 	entry = &__start_eh_frame;
 
+	dwarf_frame_cachep = kmem_cache_create("dwarf_frames",
+			sizeof(struct dwarf_frame), 0, SLAB_PANIC, NULL);
+	dwarf_reg_cachep = kmem_cache_create("dwarf_regs",
+			sizeof(struct dwarf_reg), 0, SLAB_PANIC, NULL);
+
+	dwarf_frame_pool = mempool_create(DWARF_FRAME_MIN_REQ,
+					  mempool_alloc_slab,
+					  mempool_free_slab,
+					  dwarf_frame_cachep);
+
+	dwarf_reg_pool = mempool_create(DWARF_REG_MIN_REQ,
+					 mempool_alloc_slab,
+					 mempool_free_slab,
+					 dwarf_reg_cachep);
+
 	while ((char *)entry < __stop_eh_frame) {
 		p = entry;
 
@@ -860,7 +942,7 @@ void dwarf_unwinder_init(void)
 			else
 				c_entries++;
 		} else {
-			err = dwarf_parse_fde(entry, entry_type, p, len);
+			err = dwarf_parse_fde(entry, entry_type, p, len, end);
 			if (err < 0)
 				goto out;
 			else
@@ -877,9 +959,11 @@ void dwarf_unwinder_init(void)
 	if (err)
 		goto out;
 
-	return;
+	return 0;
 
 out:
 	printk(KERN_ERR "Failed to initialise DWARF unwinder: %d\n", err);
 	dwarf_unwinder_cleanup();
+	return -EINVAL;
 }
+early_initcall(dwarf_unwinder_init);
diff --git a/arch/sh/kernel/irq.c b/arch/sh/kernel/irq.c
index 2bb43dc..278c68c 100644
--- a/arch/sh/kernel/irq.c
+++ b/arch/sh/kernel/irq.c
@@ -14,7 +14,6 @@
 #include <asm/processor.h>
 #include <asm/machvec.h>
 #include <asm/uaccess.h>
-#include <asm/dwarf.h>
 #include <asm/thread_info.h>
 #include <cpu/mmu_context.h>
 
@@ -262,9 +261,6 @@ void __init init_IRQ(void)
 		sh_mv.mv_init_irq();
 
 	irq_ctx_init(smp_processor_id());
-
-	/* This needs to be early, but not too early.. */
-	dwarf_unwinder_init();
 }
 
 #ifdef CONFIG_SPARSE_IRQ
diff --git a/arch/sh/kernel/traps.c b/arch/sh/kernel/traps.c
index b3e0067..f69bd96 100644
--- a/arch/sh/kernel/traps.c
+++ b/arch/sh/kernel/traps.c
@@ -5,18 +5,32 @@
 #include <linux/signal.h>
 #include <linux/sched.h>
 #include <linux/uaccess.h>
+#include <asm/unwinder.h>
 #include <asm/system.h>
 
 #ifdef CONFIG_BUG
-static void handle_BUG(struct pt_regs *regs)
+void handle_BUG(struct pt_regs *regs)
 {
+	const struct bug_entry *bug;
+	unsigned long bugaddr = regs->pc;
 	enum bug_trap_type tt;
-	tt = report_bug(regs->pc, regs);
+
+	if (!is_valid_bugaddr(bugaddr))
+		goto invalid;
+
+	bug = find_bug(bugaddr);
+
+	/* Switch unwinders when unwind_stack() is called */
+	if (bug->flags & BUGFLAG_UNWINDER)
+		unwinder_faulted = 1;
+
+	tt = report_bug(bugaddr, regs);
 	if (tt == BUG_TRAP_TYPE_WARN) {
-		regs->pc += instruction_size(regs->pc);
+		regs->pc += instruction_size(bugaddr);
 		return;
 	}
 
+invalid:
 	die("Kernel BUG", regs, TRAPA_BUG_OPCODE & 0xff);
 }
 
@@ -28,8 +42,10 @@ int is_valid_bugaddr(unsigned long addr)
 		return 0;
 	if (probe_kernel_address((insn_size_t *)addr, opcode))
 		return 0;
+	if (opcode == TRAPA_BUG_OPCODE)
+		return 1;
 
-	return opcode == TRAPA_BUG_OPCODE;
+	return 0;
 }
 #endif
 
diff --git a/arch/sh/kernel/traps_32.c b/arch/sh/kernel/traps_32.c
index 5634264..05a04b6 100644
--- a/arch/sh/kernel/traps_32.c
+++ b/arch/sh/kernel/traps_32.c
@@ -136,6 +136,7 @@ static void die_if_no_fixup(const char * str, struct pt_regs * regs, long err)
 			regs->pc = fixup->fixup;
 			return;
 		}
+
 		die(str, regs, err);
 	}
 }
diff --git a/arch/sh/kernel/unwinder.c b/arch/sh/kernel/unwinder.c
index 2b30fa2..468889d 100644
--- a/arch/sh/kernel/unwinder.c
+++ b/arch/sh/kernel/unwinder.c
@@ -11,6 +11,7 @@
 #include <linux/errno.h>
 #include <linux/list.h>
 #include <linux/spinlock.h>
+#include <linux/module.h>
 #include <asm/unwinder.h>
 #include <asm/atomic.h>
 
@@ -53,8 +54,6 @@ static struct list_head unwinder_list = {
 
 static DEFINE_SPINLOCK(unwinder_lock);
 
-static atomic_t unwinder_running = ATOMIC_INIT(0);
-
 /**
  * select_unwinder - Select the best registered stack unwinder.
  *
@@ -122,6 +121,8 @@ int unwinder_register(struct unwinder *u)
 	return ret;
 }
 
+int unwinder_faulted = 0;
+
 /*
  * Unwind the call stack and pass information to the stacktrace_ops
  * functions. Also handle the case where we need to switch to a new
@@ -144,19 +145,20 @@ void unwind_stack(struct task_struct *task, struct pt_regs *regs,
 	 * 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) {
+	if (unwinder_faulted) {
 		spin_lock_irqsave(&unwinder_lock, flags);
 
-		if (!list_is_singular(&unwinder_list)) {
+		/* Make sure no one beat us to changing the unwinder */
+		if (unwinder_faulted && !list_is_singular(&unwinder_list)) {
 			list_del(&curr_unwinder->list);
 			curr_unwinder = select_unwinder();
+
+			unwinder_faulted = 0;
 		}
 
 		spin_unlock_irqrestore(&unwinder_lock, flags);
-		atomic_dec(&unwinder_running);
 	}
 
 	curr_unwinder->dump(task, regs, sp, ops, data);
-
-	atomic_dec(&unwinder_running);
 }
+EXPORT_SYMBOL_GPL(unwind_stack);