Merge branch 'sh/dwarf-unwinder' of git://github.com/mfleming/linux-2.6 into sh/dwarf-unwinder

arch/sh/include/asm/bug.h

@@ -1,6 +1,7 @@
 #ifndef __ASM_SH_BUG_H
 #define __ASM_SH_BUG_H
 
+#define TRAPA_UNWINDER_BUG_OPCODE 0xc33b /* trapa #0x3b */
 #define TRAPA_BUG_OPCODE	0xc33e	/* trapa #0x3e */
 
 #ifdef CONFIG_GENERIC_BUG
@@ -72,6 +73,30 @@ do {							\
 	unlikely(__ret_warn_on);				\
 })
 
+#define UNWINDER_BUG()						\
+do {							\
+	__asm__ __volatile__ (				\
+		"1:\t.short %O0\n"			\
+		_EMIT_BUG_ENTRY		\
+		 :					\
+		 : "n" (TRAPA_UNWINDER_BUG_OPCODE),	\
+		   "i" (__FILE__),			\
+		   "i" (__LINE__), "i" (0),		\
+		   "i" (sizeof(struct bug_entry)));	\
+} while (0)
+
+#define UNWINDER_BUG_ON(x) ({					\
+	int __ret_unwinder_on = !!(x);				\
+	if (__builtin_constant_p(__ret_unwinder_on)) {		\
+		if (__ret_unwinder_on)				\
+			UNWINDER_BUG();				\
+	} else {						\
+		if (unlikely(__ret_unwinder_on))		\
+			UNWINDER_BUG();				\
+	}							\
+	unlikely(__ret_unwinder_on);				\
+})
+
 #endif /* CONFIG_GENERIC_BUG */
 
 #include <asm-generic/bug.h>

arch/sh/include/asm/dwarf.h

@@ -265,10 +265,7 @@ struct dwarf_frame {
 
 	unsigned long pc;
 
-	struct dwarf_reg *regs;
-	unsigned int num_regs;	/* how many regs are allocated? */
-
-	unsigned int depth;	/* what level are we in the callstack? */
+	struct list_head reg_list;
 
 	unsigned long cfa;
 
@@ -292,20 +289,15 @@ struct dwarf_frame {
  *	@flags: Describes how to calculate the value of this register
  */
 struct dwarf_reg {
+	struct list_head link;
+
+	unsigned int number;
+
 	unsigned long addr;
 	unsigned long flags;
 #define DWARF_REG_OFFSET	(1 << 0)
-};
-
-/**
- *	dwarf_stack - a DWARF stack contains a collection of DWARF frames
- *	@depth: the number of frames in the stack
- *	@level: an array of DWARF frames, indexed by stack level
- *
- */
-struct dwarf_stack {
-	unsigned int depth;
-	struct dwarf_frame **level;
+#define DWARF_VAL_OFFSET	(1 << 1)
+#define DWARF_UNDEFINED		(1 << 2)
 };
 
 /*
@@ -372,13 +364,14 @@ static inline unsigned int DW_CFA_operand(unsigned long insn)
 
 extern struct dwarf_frame *dwarf_unwind_stack(unsigned long,
 					      struct dwarf_frame *);
-#endif /* __ASSEMBLY__ */
+#endif /* !__ASSEMBLY__ */
 
 #define CFI_STARTPROC	.cfi_startproc
 #define CFI_ENDPROC	.cfi_endproc
 #define CFI_DEF_CFA	.cfi_def_cfa
 #define CFI_REGISTER	.cfi_register
 #define CFI_REL_OFFSET	.cfi_rel_offset
+#define CFI_UNDEFINED	.cfi_undefined
 
 #else
 
@@ -392,6 +385,7 @@ extern struct dwarf_frame *dwarf_unwind_stack(unsigned long,
 #define CFI_DEF_CFA	CFI_IGNORE
 #define CFI_REGISTER	CFI_IGNORE
 #define CFI_REL_OFFSET	CFI_IGNORE
+#define CFI_UNDEFINED	CFI_IGNORE
 
 #ifndef __ASSEMBLY__
 static inline void dwarf_unwinder_init(void)

arch/sh/include/asm/system.h

@@ -181,6 +181,11 @@ BUILD_TRAP_HANDLER(breakpoint);
 BUILD_TRAP_HANDLER(singlestep);
 BUILD_TRAP_HANDLER(fpu_error);
 BUILD_TRAP_HANDLER(fpu_state_restore);
+BUILD_TRAP_HANDLER(unwinder);
+
+#ifdef CONFIG_BUG
+extern void handle_BUG(struct pt_regs *);
+#endif
 
 #define arch_align_stack(x) (x)
 

arch/sh/include/asm/unwinder.h

@@ -22,4 +22,10 @@ extern void stack_reader_dump(struct task_struct *, struct pt_regs *,
 			      unsigned long *, const struct stacktrace_ops *,
 			      void *);
 
+/*
+ * Used by fault handling code to signal to the unwinder code that it
+ * should switch to a different unwinder.
+ */
+extern int unwinder_faulted;
+
 #endif /* _LINUX_UNWINDER_H */

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
+
 	! Setup return address and jump to do_IRQ
 	mov.l	4f, r9		! fetch return address
 	lds	r9, pr		! put return address in pr

arch/sh/kernel/debugtraps.S

@@ -19,6 +19,10 @@
 
 #if !defined(CONFIG_SH_STANDARD_BIOS)
 #define sh_bios_handler			debug_trap_handler
+#endif
+
+#if !defined(CONFIG_DWARF_UNWINDER)
+#define unwinder_trap_handler		debug_trap_handler
 #endif
 
 	.data
@@ -35,7 +39,7 @@ ENTRY(debug_trap_table)
 	.long debug_trap_handler	/* 0x38 */
 	.long debug_trap_handler	/* 0x39 */
 	.long debug_trap_handler	/* 0x3a */
-	.long debug_trap_handler	/* 0x3b */
+	.long unwinder_trap_handler	/* 0x3b */
 	.long breakpoint_trap_handler	/* 0x3c */
 	.long singlestep_trap_handler	/* 0x3d */
 	.long bug_trap_handler		/* 0x3e */

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>
@@ -25,6 +27,17 @@
 #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);
 static DEFINE_SPINLOCK(dwarf_cie_lock);
 
@@ -33,47 +46,70 @@ 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;
+	struct dwarf_reg *reg;
 
-	new_size = num_regs * sizeof(*regs);
-	old_size = frame->num_regs * sizeof(*regs);
-
-	/* 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();
+	}
+
+	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);
 	}
+}
+
+/**
+ *	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;
 
-	if (frame->regs) {
-		memcpy(regs, frame->regs, old_size);
-		kfree(frame->regs);
+	list_for_each_entry(reg, &frame->reg_list, link) {
+		if (reg->number == reg_num)
+			return reg;
 	}
 
-	frame->regs = regs;
-	frame->num_regs = num_regs;
+	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;
@@ -347,6 +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;
 
@@ -369,9 +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:
@@ -415,6 +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,
@@ -453,17 +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);
@@ -474,12 +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;
 		}
 	}
@@ -496,14 +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
@@ -516,11 +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) {
@@ -540,7 +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);
@@ -560,10 +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 {
@@ -580,27 +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,
@@ -625,7 +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;
@@ -655,7 +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++;
@@ -683,16 +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;
 		}
 	}
@@ -709,7 +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;
@@ -756,7 +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);
@@ -766,17 +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 = dwarf_unwind_stack(0, NULL);
+	_frame = NULL;
+	return_addr = 0;
 
-	while (frame && frame->return_addr) {
-		ops->address(data, frame->return_addr, 1);
-		frame = frame->next;
+	while (1) {
+		frame = dwarf_unwind_stack(return_addr, _frame);
+
+		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);
 	}
 }
 
@@ -801,6 +869,9 @@ static void dwarf_unwinder_cleanup(void)
 
 	list_for_each_entry(fde, &dwarf_fde_list, link)
 		kfree(fde);
+
+	kmem_cache_destroy(dwarf_reg_cachep);
+	kmem_cache_destroy(dwarf_frame_cachep);
 }
 
 /**
@@ -827,6 +898,21 @@ static int __init 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;
 
@@ -856,7 +942,7 @@ static int __init 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

arch/sh/kernel/traps.c

@@ -8,7 +8,7 @@
 #include <asm/system.h>
 
 #ifdef CONFIG_BUG
-static void handle_BUG(struct pt_regs *regs)
+void handle_BUG(struct pt_regs *regs)
 {
 	enum bug_trap_type tt;
 	tt = report_bug(regs->pc, regs);
@@ -29,7 +29,10 @@ int is_valid_bugaddr(unsigned long addr)
 	if (probe_kernel_address((insn_size_t *)addr, opcode))
 		return 0;
 
-	return opcode == TRAPA_BUG_OPCODE;
+	if (opcode == TRAPA_BUG_OPCODE || opcode == TRAPA_UNWINDER_BUG_OPCODE)
+		return 1;
+
+	return 0;
 }
 #endif
 

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);
 	}
 }

arch/sh/kernel/unwinder.c

@@ -54,8 +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.
  *
@@ -123,6 +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
@@ -145,20 +145,41 @@ 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);
+}
+
+/*
+ * Trap handler for UWINDER_BUG() statements. We must switch to the
+ * unwinder with the next highest rating.
+ */
+BUILD_TRAP_HANDLER(unwinder)
+{
+	insn_size_t insn;
+	TRAP_HANDLER_DECL;
+
+	/* Rewind */
+	regs->pc -= instruction_size(ctrl_inw(regs->pc - 4));
+	insn = *(insn_size_t *)instruction_pointer(regs);
+
+	/* Switch unwinders when unwind_stack() is called */
+	unwinder_faulted = 1;
 
-	atomic_dec(&unwinder_running);
+#ifdef CONFIG_BUG
+	handle_BUG(regs);
+#endif
 }
 EXPORT_SYMBOL_GPL(unwind_stack);