From 967cc5371113f9806b39a2ebb2174af2883d96fe Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Thu, 9 Jul 2009 23:28:49 +0200 Subject: [PATCH] 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 --- lib/is_single_threaded.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/lib/is_single_threaded.c b/lib/is_single_threaded.c index 434010980bd..bd2bea96336 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); 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; }