[PATCH 6/8] blackfin: Fix possible race on task->mm

[Anton Vorontsov <anton.vorontsov@linaro.org>]

Even when in atomic context, grabbing irqsave variant of tasklist lock
is not enough to protect task->mm from disappearing on SMP machines.
Instead, we have to grab the task lock.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---

p.s. I don't have blackfin toolchain handy, so I did not build-test
the patch.

 arch/blackfin/kernel/trace.c |   21 +++++++++------------
 1 files changed, 9 insertions(+), 12 deletions(-)

diff --git a/arch/blackfin/kernel/trace.c b/arch/blackfin/kernel/trace.c
index 050db44..19bc02a 100644
--- a/arch/blackfin/kernel/trace.c
+++ b/arch/blackfin/kernel/trace.c
@@ -10,6 +10,7 @@
 #include <linux/hardirq.h>
 #include <linux/thread_info.h>
 #include <linux/mm.h>
+#include <linux/sched.h>
 #include <linux/uaccess.h>
 #include <linux/module.h>
 #include <linux/kallsyms.h>
@@ -27,7 +28,6 @@ void decode_address(char *buf, unsigned long address)
 	struct task_struct *p;
 	struct mm_struct *mm;
 	unsigned long flags, offset;
-	unsigned char in_atomic = (bfin_read_IPEND() & 0x10) || in_atomic();
 	struct rb_node *n;
 
 #ifdef CONFIG_KALLSYMS
@@ -113,15 +113,13 @@ void decode_address(char *buf, unsigned long address)
 	 */
 	write_lock_irqsave(&tasklist_lock, flags);
 	for_each_process(p) {
-		mm = (in_atomic ? p->mm : get_task_mm(p));
+		task_lock(p);
+		mm = p->mm;
 		if (!mm)
-			continue;
+			goto __continue;
 
-		if (!down_read_trylock(&mm->mmap_sem)) {
-			if (!in_atomic)
-				mmput(mm);
-			continue;
-		}
+		if (!down_read_trylock(&mm->mmap_sem))
+			goto __continue;
 
 		for (n = rb_first(&mm->mm_rb); n; n = rb_next(n)) {
 			struct vm_area_struct *vma;
@@ -163,8 +161,7 @@ void decode_address(char *buf, unsigned long address)
 						name, vma->vm_start, vma->vm_end);
 
 				up_read(&mm->mmap_sem);
-				if (!in_atomic)
-					mmput(mm);
+				task_unlock(p);
 
 				if (buf[0] == '\0')
 					sprintf(buf, "[ %s ] dynamic memory", name);
@@ -174,8 +171,8 @@ void decode_address(char *buf, unsigned long address)
 		}
 
 		up_read(&mm->mmap_sem);
-		if (!in_atomic)
-			mmput(mm);
+__continue:
+		task_unlock(p);
 	}
 
 	/*
-- 
1.7.7.6