[PATCH] clarify get_task_mm (mmgrab)

Clarify mmgrab by collapsing it into get_task_mm (in fork.c not inline),
and commenting on the special case it is guarding against: when use_mm in
an AIO daemon temporarily adopts the mm while it's on its way out.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/proc/array.c

@@ -316,11 +316,7 @@ int proc_pid_stat(struct task_struct *task, char * buffer)
 
 	state = *get_task_state(task);
 	vsize = eip = esp = 0;
-	task_lock(task);
-	mm = task->mm;
-	if(mm)
-		mm = mmgrab(mm);
-	task_unlock(task);
+	mm = get_task_mm(task);
 	if (mm) {
 		down_read(&mm->mmap_sem);
 		vsize = task_vsize(mm);

include/linux/sched.h

@@ -788,8 +788,8 @@ static inline void mmdrop(struct mm_struct * mm)
 
 /* mmput gets rid of the mappings and all user-space */
 extern void mmput(struct mm_struct *);
-/* Grab a reference to the mm if its not already going away */
-extern struct mm_struct *mmgrab(struct mm_struct *);
+/* Grab a reference to a task's mm, if it is not already going away */
+extern struct mm_struct *get_task_mm(struct task_struct *task);
 /* Remove the current tasks stale references to the old mm_struct */
 extern void mm_release(struct task_struct *, struct mm_struct *);
 
@@ -891,26 +891,6 @@ static inline void task_unlock(struct task_struct *p)
 	spin_unlock(&p->alloc_lock);
 }
 
-/**
- * get_task_mm - acquire a reference to the task's mm
- *
- * Returns %NULL if the task has no mm. User must release
- * the mm via mmput() after use.
- */
-static inline struct mm_struct * get_task_mm(struct task_struct * task)
-{
-	struct mm_struct * mm;
- 
-	task_lock(task);
-	mm = task->mm;
-	if (mm)
-		mm = mmgrab(mm);
-	task_unlock(task);
-
-	return mm;
-}
- 
- 
 /* set thread flags in other task's structures
  * - see asm/thread_info.h for TIF_xxxx flags available
  */

kernel/fork.c

@@ -477,20 +477,34 @@ void mmput(struct mm_struct *mm)
 	}
 }
 
-/*
- * Checks if the use count of an mm is non-zero and if so
- * returns a reference to it after bumping up the use count.
- * If the use count is zero, it means this mm is going away,
- * so return NULL.
+/**
+ * get_task_mm - acquire a reference to the task's mm
+ *
+ * Returns %NULL if the task has no mm.  Checks if the use count
+ * of the mm is non-zero and if so returns a reference to it, after
+ * bumping up the use count.  User must release the mm via mmput()
+ * after use.  Typically used by /proc and ptrace.
+ *
+ * If the use count is zero, it means that this mm is going away,
+ * so return %NULL.  This only happens in the case of an AIO daemon
+ * which has temporarily adopted an mm (see use_mm), in the course
+ * of its final mmput, before exit_aio has completed.
  */
-struct mm_struct *mmgrab(struct mm_struct *mm)
+struct mm_struct *get_task_mm(struct task_struct *task)
 {
+	struct mm_struct *mm;
+
+	task_lock(task);
+	mm = task->mm;
+	if (mm) {
 		spin_lock(&mmlist_lock);
 		if (!atomic_read(&mm->mm_users))
 			mm = NULL;
 		else
 			atomic_inc(&mm->mm_users);
 		spin_unlock(&mmlist_lock);
+	}
+	task_unlock(task);
 	return mm;
 }