[PATCH 1/2] mm_for_maps: shift down_read(mmap_sem) to the caller

mm_for_maps() takes ->mmap_sem after security checks, this looks
strange and obfuscates the locking rules. Move this lock to its
single caller, m_start().

Signed-off-by: Oleg Nesterov <oleg@...>
---

 fs/proc/task_mmu.c   |    1 +
 fs/proc/task_nommu.c |    1 +
 fs/proc/base.c       |    8 +++-----
 3 files changed, 5 insertions(+), 5 deletions(-)

--- WAIT/fs/proc/task_mmu.c~1_MOVE_MMAP_SEM 2009-05-03 17:15:27.000000000 +0200
+++ WAIT/fs/proc/task_mmu.c 2009-07-10 02:00:56.000000000 +0200
@@ -119,6 +119,7 @@ static void *m_start(struct seq_file *m,
  mm = mm_for_maps(priv->task);
  if (!mm)
  return NULL;
+ down_read(&mm->mmap_sem);
 
  tail_vma = get_gate_vma(priv->task);
  priv->tail_vma = tail_vma;
--- WAIT/fs/proc/task_nommu.c~1_MOVE_MMAP_SEM 2009-04-09 00:39:09.000000000 +0200
+++ WAIT/fs/proc/task_nommu.c 2009-07-10 02:01:25.000000000 +0200
@@ -189,6 +189,7 @@ static void *m_start(struct seq_file *m,
  priv->task = NULL;
  return NULL;
  }
+ down_read(&mm->mmap_sem);
 
  /* start from the Nth VMA */
  for (p = rb_first(&mm->mm_rb); p; p = rb_next(p))
--- WAIT/fs/proc/base.c~1_MOVE_MMAP_SEM 2009-07-09 17:56:27.000000000 +0200
+++ WAIT/fs/proc/base.c 2009-07-10 02:05:14.000000000 +0200
@@ -235,9 +235,8 @@ static int check_mem_permission(struct t
 struct mm_struct *mm_for_maps(struct task_struct *task)
 {
  struct mm_struct *mm = get_task_mm(task);
- if (!mm)
- return NULL;
- if (mm != current->mm) {
+
+ if (mm && mm != current->mm) {
  /*
  * task->mm can be changed before security check,
  * in that case we must notice the change after.
@@ -245,10 +244,9 @@ struct mm_struct *mm_for_maps(struct tas
  if (!ptrace_may_access(task, PTRACE_MODE_READ) ||
     mm != task->mm) {
  mmput(mm);
- return NULL;
+ mm = NULL;
  }
  }
- down_read(&mm->mmap_sem);
  return mm;
 }
 

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