[PATCH v2 -mm 2/2] current_is_single_threaded: don't use ->mmap_sem

Oleg Nesterov
2009-07-10T01:53:15Z

current_is_single_threaded() can safely miss a freshly forked CLONE_VM
task, but in this case it must not miss its parent. That is why we take
mm->mmap_sem for writing to make sure a thread/task with the same ->mm
can't pass exit_mm() and disappear.

However we can avoid ->mmap_sem and rely on rcu/barriers:

- if we do not see the exiting parent on thread/process list
we see the result of list_del_rcu(), in this case we must
also see the result of list_add_rcu() which does wmb().

- if we do see the parent but its ->mm == NULL, we need rmb()
to make sure we can't miss the child.

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

lib/is_single_threaded.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

--- WAIT/lib/is_single_threaded.c~ISS_2_RCU 2009-07-09 22:16:29.000000000 +0200
+++ WAIT/lib/is_single_threaded.c 2009-07-09 22:54:41.000000000 +0200
@@ -22,8 +22,6 @@ bool current_is_single_threaded(void)
struct task_struct *p, *t;
bool ret;

- might_sleep();
-
if (atomic_read(&task->signal->live) != 1)
return false;

@@ -31,7 +29,6 @@ bool current_is_single_threaded(void)
return true;

ret = false;
- down_write(&mm->mmap_sem);
rcu_read_lock();
for_each_process(p) {
if (unlikely(p->flags & PF_KTHREAD))
@@ -45,12 +42,17 @@ bool current_is_single_threaded(void)
goto found;
if (likely(t->mm))
break;
+ /*
+ * t->mm == NULL. Make sure next_thread/next_task
+ * will see other CLONE_VM tasks which might be
+ * forked before exiting.
+ */
+ smp_rmb();
} while_each_thread(p, t);
}
ret = true;
found:
rcu_read_unlock();
- up_write(&mm->mmap_sem);

return ret;
}