oprofile: reduce mmap_sem hold for mm->exe_file

sync_buffer() needs the mmap_sem for two distinct operations, both only
occurring upon user context switch handling:

1) Dealing with the exe_file.

2) Adding the dcookie data as we need to lookup the vma that
backs it. This is done via add_sample() and add_data().

This patch isolates 1), for it will no longer need the mmap_sem for
serialization. However, for now, make of the more standard
get_mm_exe_file(), requiring only holding the mmap_sem to read the value,
and relying on reference counting to make sure that the exe file won't
dissappear underneath us while doing the get dcookie.

As a consequence, for 2) we move the mmap_sem locking into where we really
need it, in lookup_dcookie(). The benefits are twofold: reduce mmap_sem
hold times, and cleaner code.

[[e-mail]: export get_mm_exe_file for arch/x86/oprofile/oprofile.ko]
Signed-off-by: Davidlohr Bueso <[e-mail]>
Cc: Robert Richter <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/oprofile/buffer_sync.c

···
  * objects.
  */
 
+#include <linux/file.h>
 #include <linux/mm.h>
 #include <linux/workqueue.h>
 #include <linux/notifier.h>
···
 static unsigned long get_exec_dcookie(struct mm_struct *mm)
 {
 	unsigned long cookie = NO_COOKIE;
+	struct file *exe_file;
 
-	if (mm && mm->exe_file)
-		cookie = fast_get_dcookie(&mm->exe_file->f_path);
+	if (!mm)
+		goto done;
 
+	exe_file = get_mm_exe_file(mm);
+	if (!exe_file)
+		goto done;
+
+	cookie = fast_get_dcookie(&exe_file->f_path);
+	fput(exe_file);
+done:
 	return cookie;
 }
 
···
  * pair that can then be added to the global event buffer. We make
  * sure to do this lookup before a mm->mmap modification happens so
  * we don't lose track.
+ *
+ * The caller must ensure the mm is not nil (ie: not a kernel thread).
  */
 static unsigned long
 lookup_dcookie(struct mm_struct *mm, unsigned long addr, off_t *offset)
···
 	unsigned long cookie = NO_COOKIE;
 	struct vm_area_struct *vma;
 
+	down_read(&mm->mmap_sem);
 	for (vma = find_vma(mm, addr); vma; vma = vma->vm_next) {
 
 		if (addr < vma->vm_start || addr >= vma->vm_end)
···
 
 	if (!vma)
 		cookie = INVALID_COOKIE;
+	up_read(&mm->mmap_sem);
 
 	return cookie;
 }
···
 {
 	if (!mm)
 		return;
-	up_read(&mm->mmap_sem);
 	mmput(mm);
 }
-
-
-static struct mm_struct *take_tasks_mm(struct task_struct *task)
-{
-	struct mm_struct *mm = get_task_mm(task);
-	if (mm)
-		down_read(&mm->mmap_sem);
-	return mm;
-}
-
 
 static inline int is_code(unsigned long val)
 {
···
 				new = (struct task_struct *)val;
 				oldmm = mm;
 				release_mm(oldmm);
-				mm = take_tasks_mm(new);
+				mm = get_task_mm(new);
 				if (mm != oldmm)
 					cookie = get_exec_dcookie(mm);
 				add_user_ctx_switch(new, cookie);

kernel/fork.c

···
 	rcu_read_unlock();
 	return exe_file;
 }
+EXPORT_SYMBOL(get_mm_exe_file);
 
 /**
  * get_task_mm - acquire a reference to the task's mm