blackfin: a couple of task->mm handling fixes

The patch fixes two problems:

1. Working with task->mm w/o getting mm or grabing the task lock is
   dangerous as ->mm might disappear (exit_mm() assigns NULL under
   task_lock(), so tasklist lock is not enough).

   We can't use get_task_mm()/mmput() pair as mmput() might sleep,
   so we have to take the task lock while handle its mm.

2. Checking for process->mm is not enough because process' main
   thread may exit or detach its mm via use_mm(), but other threads
   may still have a valid mm.

   To catch this we use find_lock_task_mm(), which walks up all
   threads and returns an appropriate task (with task lock held).

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Cc: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 13 deletions

diff --git a/arch/blackfin/kernel/trace.c b/arch/blackfin/kernel/trace.c
index 44bbf2f564cb..d08f0e3e2dcc 100644
--- a/arch/blackfin/kernel/trace.c
+++ b/arch/blackfin/kernel/trace.c
@@ -10,6 +10,8 @@
 #include <linux/hardirq.h>
 #include <linux/thread_info.h>
 #include <linux/mm.h>
+#include <linux/oom.h>
+#include <linux/sched.h>
 #include <linux/uaccess.h>
 #include <linux/module.h>
 #include <linux/kallsyms.h>
@@ -28,7 +30,6 @@ 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
@@ -114,15 +115,15 @@ 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));
-                if (!mm)
-                        continue;
+                struct task_struct *t;
 
-                if (!down_read_trylock(&mm->mmap_sem)) {
-                        if (!in_atomic)
-                                mmput(mm);
+                t = find_lock_task_mm(p);
+                if (!t)
                         continue;
-                }
+
+                mm = t->mm;
+                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;
@@ -131,7 +132,7 @@ void decode_address(char *buf, unsigned long address)
 
                         if (address >= vma->vm_start && address < vma->vm_end) {
                                 char _tmpbuf[256];
-                                char *name = p->comm;
+                                char *name = t->comm;
                                 struct file *file = vma->vm_file;
 
                                 if (file) {
@@ -164,8 +165,7 @@ 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(t);
 
                                 if (buf[0] == '\0')
                                         sprintf(buf, "[ %s ] dynamic memory", name);
@@ -175,8 +175,8 @@ void decode_address(char *buf, unsigned long address)
                 }
 
                 up_read(&mm->mmap_sem);
-                if (!in_atomic)
-                        mmput(mm);
+__continue:
+                task_unlock(t);
         }
 
         /*