From: Jeff Dike <jdike@addtoit.com>

Implement get_wchan - the algorithm is similar to x86.  It starts with the
stack pointer of the process in question and looks above that for addresses
that are kernel text.  The second one which isn't in the scheduler is the one
that's returned.  The first one is ignored because that will be UML's own
context switching routine.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 arch/um/kernel/process.c           |   37 +++++++++++++++++++++++++++
 include/asm-um/processor-generic.h |    2 -
 2 files changed, 38 insertions(+), 1 deletion(-)

diff -puN arch/um/kernel/process.c~uml-implement-get_wchan arch/um/kernel/process.c
--- a/arch/um/kernel/process.c~uml-implement-get_wchan
+++ a/arch/um/kernel/process.c
@@ -459,3 +459,40 @@ unsigned long arch_align_stack(unsigned 
 	return sp & ~0xf;
 }
 #endif
+
+unsigned long get_wchan(struct task_struct *p)
+{
+	unsigned long stack_page, sp, ip, count = 0;
+
+	if ((p == NULL) || (p == current) || (p->state == TASK_RUNNING))
+		return 0;
+
+	stack_page = (unsigned long) task_stack_page(p);
+	/* Bail if the process has no kernel stack for some reason */
+	if (stack_page == 0)
+		return 0;
+
+	sp = p->thread.switch_buf->JB_SP;
+	/*
+	 * Bail if the stack pointer is below the bottom of the kernel
+	 * stack for some reason
+	 */
+	if (sp < stack_page)
+		return 0;
+
+	while (sp < stack_page + THREAD_SIZE) {
+		ip = *((unsigned long *) sp);
+		if (kernel_text_address(ip) && !in_sched_functions(ip)) {
+			/*
+			 * Skip one valid IP, which will be the low-level UML
+			 * context switcher.
+			 */
+			if (count++ == 1)
+				return ip;
+		}
+
+		sp += sizeof(unsigned long);
+	}
+
+	return 0;
+}
diff -puN include/asm-um/processor-generic.h~uml-implement-get_wchan include/asm-um/processor-generic.h
--- a/include/asm-um/processor-generic.h~uml-implement-get_wchan
+++ a/include/asm-um/processor-generic.h
@@ -128,6 +128,6 @@ extern struct cpuinfo_um cpu_data[];
 
 
 #define KSTK_REG(tsk, reg) get_thread_reg(reg, &tsk->thread.switch_buf)
-#define get_wchan(p) (0)
+extern unsigned long get_wchan(struct task_struct *p);
 
 #endif
_