[PATCH 09/19] x86/dumpstack: simplify in_exception_stack()

[Josh Poimboeuf]

in_exception_stack() does some bad, bad things just so the unwinder can
print different values for different areas of the debug exception stack.

There's no need to clarify where exactly on the stack it is.  Just print
"#DB" and be done with it.

Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 arch/x86/kernel/dumpstack_64.c | 106 +++++++++++++----------------------------
 1 file changed, 32 insertions(+), 74 deletions(-)

diff --git a/arch/x86/kernel/dumpstack_64.c b/arch/x86/kernel/dumpstack_64.c
index 634ed22..0641d75 100644
--- a/arch/x86/kernel/dumpstack_64.c
+++ b/arch/x86/kernel/dumpstack_64.c
@@ -16,83 +16,41 @@
 
 #include <asm/stacktrace.h>
 
+static char *exception_stack_names[N_EXCEPTION_STACKS] = {
+		[ DOUBLEFAULT_STACK-1	]	= "#DF",
+		[ NMI_STACK-1		]	= "NMI",
+		[ DEBUG_STACK-1		]	= "#DB",
+		[ MCE_STACK-1		]	= "#MC",
+};
 
-#define N_EXCEPTION_STACKS_END \
-		(N_EXCEPTION_STACKS + DEBUG_STKSZ/EXCEPTION_STKSZ - 2)
-
-static char x86_stack_ids[][8] = {
-		[ DEBUG_STACK-1			]	= "#DB",
-		[ NMI_STACK-1			]	= "NMI",
-		[ DOUBLEFAULT_STACK-1		]	= "#DF",
-		[ MCE_STACK-1			]	= "#MC",
-#if DEBUG_STKSZ > EXCEPTION_STKSZ
-		[ N_EXCEPTION_STACKS ...
-		  N_EXCEPTION_STACKS_END	]	= "#DB[?]"
-#endif
+static unsigned long exception_stack_sizes[N_EXCEPTION_STACKS] = {
+	[0 ... N_EXCEPTION_STACKS - 1]		= EXCEPTION_STKSZ,
+	[DEBUG_STACK - 1]			= DEBUG_STKSZ
 };
 
-static unsigned long *in_exception_stack(unsigned long stack, unsigned *usedp,
-					 char **idp)
+static unsigned long *in_exception_stack(unsigned long *s, char **name,
+					 unsigned long *visit_mask)
 {
+	unsigned long stack = (unsigned long)s;
+	unsigned long begin, end;
 	unsigned k;
 
-	/*
-	 * Iterate over all exception stacks, and figure out whether
-	 * 'stack' is in one of them:
-	 */
+	BUILD_BUG_ON(N_EXCEPTION_STACKS != 4);
+
 	for (k = 0; k < N_EXCEPTION_STACKS; k++) {
-		unsigned long end = this_cpu_ptr(&orig_ist)->ist[k];
-		/*
-		 * Is 'stack' above this exception frame's end?
-		 * If yes then skip to the next frame.
-		 */
-		if (stack >= end)
+		end   = this_cpu_ptr(&orig_ist)->ist[k];
+		begin = end - exception_stack_sizes[k];
+
+		if (stack < begin || stack >= end)
 			continue;
-		/*
-		 * Is 'stack' above this exception frame's start address?
-		 * If yes then we found the right frame.
-		 */
-		if (stack >= end - EXCEPTION_STKSZ) {
-			/*
-			 * Make sure we only iterate through an exception
-			 * stack once. If it comes up for the second time
-			 * then there's something wrong going on - just
-			 * break out and return NULL:
-			 */
-			if (*usedp & (1U << k))
-				break;
-			*usedp |= 1U << k;
-			*idp = x86_stack_ids[k];
-			return (unsigned long *)end;
-		}
-		/*
-		 * If this is a debug stack, and if it has a larger size than
-		 * the usual exception stacks, then 'stack' might still
-		 * be within the lower portion of the debug stack:
-		 */
-#if DEBUG_STKSZ > EXCEPTION_STKSZ
-		if (k == DEBUG_STACK - 1 && stack >= end - DEBUG_STKSZ) {
-			unsigned j = N_EXCEPTION_STACKS - 1;
 
-			/*
-			 * Black magic. A large debug stack is composed of
-			 * multiple exception stack entries, which we
-			 * iterate through now. Dont look:
-			 */
-			do {
-				++j;
-				end -= EXCEPTION_STKSZ;
-				x86_stack_ids[j][4] = '1' +
-						(j - N_EXCEPTION_STACKS);
-			} while (stack < end - EXCEPTION_STKSZ);
-			if (*usedp & (1U << j))
-				break;
-			*usedp |= 1U << j;
-			*idp = x86_stack_ids[j];
-			return (unsigned long *)end;
-		}
-#endif
+		if (test_and_set_bit(k, visit_mask))
+			return false;
+
+		*name = exception_stack_names[k];
+		return (unsigned long *)end;
 	}
+
 	return NULL;
 }
 
@@ -113,7 +71,7 @@ enum stack_type {
 static enum stack_type
 analyze_stack(struct task_struct *task, unsigned long *stack,
 	      unsigned long **stack_end, unsigned long *irq_stack,
-	      unsigned *used, char **id)
+	      unsigned long *visit_mask, char **name)
 {
 	unsigned long addr;
 
@@ -121,7 +79,7 @@ analyze_stack(struct task_struct *task, unsigned long *stack,
 	if ((unsigned long)task_stack_page(task) == addr)
 		return STACK_IS_NORMAL;
 
-	*stack_end = in_exception_stack((unsigned long)stack, used, id);
+	*stack_end = in_exception_stack(stack, name, visit_mask);
 	if (*stack_end)
 		return STACK_IS_EXCEPTION;
 
@@ -149,7 +107,7 @@ void dump_trace(struct task_struct *task, struct pt_regs *regs,
 		const struct stacktrace_ops *ops, void *data)
 {
 	unsigned long *irq_stack = (unsigned long *)this_cpu_read(irq_stack_ptr);
-	unsigned used = 0;
+	unsigned long visit_mask = 0;
 	int graph = 0;
 	int done = 0;
 
@@ -165,10 +123,10 @@ void dump_trace(struct task_struct *task, struct pt_regs *regs,
 	while (!done) {
 		unsigned long *stack_end;
 		enum stack_type stype;
-		char *id;
+		char *name;
 
-		stype = analyze_stack(task, stack, &stack_end, irq_stack, &used,
-				      &id);
+		stype = analyze_stack(task, stack, &stack_end, irq_stack,
+				      &visit_mask, &name);
 
 		/* Default finish unless specified to continue */
 		done = 1;
@@ -181,7 +139,7 @@ void dump_trace(struct task_struct *task, struct pt_regs *regs,
 
 		case STACK_IS_EXCEPTION:
 
-			if (ops->stack(data, id) < 0)
+			if (ops->stack(data, name) < 0)
 				break;
 
 			bp = ops->walk_stack(task, stack, bp, ops,
-- 
2.7.4