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kernel: is_current_single_threaded: don't use ->mmap_sem

is_current_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 <>
Acked-by: David Howells <>
Signed-off-by: James Morris <>
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commit 967cc5371113f9806b39a2ebb2174af2883d96fe 1 parent 5bb459b
@utrace utrace authored James Morris committed
Showing with 6 additions and 4 deletions.
  1. +6 −4 lib/is_single_threaded.c
10 lib/is_single_threaded.c
@@ -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);
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))
+ /*
+ * 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;
- up_write(&mm->mmap_sem);
return ret;
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