path: root/lib/is_single_threaded.c
diff options
authorOleg Nesterov <oleg@redhat.com>2009-07-09 23:28:49 +0200
committerJames Morris <jmorris@namei.org>2009-07-17 09:11:31 +1000
commit967cc5371113f9806b39a2ebb2174af2883d96fe (patch)
treed99845169cde5e238a02d1e984ff9aa65b66482f /lib/is_single_threaded.c
parent5bb459bb45d1ad3c177485dcf0af01580aa31125 (diff)
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 <oleg@redhat.com> Acked-by: David Howells <dhowells@redhat.com> Signed-off-by: James Morris <jmorris@namei.org>
Diffstat (limited to 'lib/is_single_threaded.c')
1 files changed, 6 insertions, 4 deletions
diff --git a/lib/is_single_threaded.c b/lib/is_single_threaded.c
index 434010980bdf..bd2bea963364 100644
--- a/lib/is_single_threaded.c
+++ b/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;