diff options
| author | Oleg Nesterov <oleg@redhat.com> | 2009-07-09 23:28:49 +0200 |
|---|---|---|
| committer | James Morris <jmorris@namei.org> | 2009-07-17 09:11:31 +1000 |
| commit | 967cc5371113f9806b39a2ebb2174af2883d96fe (patch) | |
| tree | d99845169cde5e238a02d1e984ff9aa65b66482f /lib | |
| parent | 5bb459bb45d1ad3c177485dcf0af01580aa31125 (diff) | |
| download | linux-967cc5371113f9806b39a2ebb2174af2883d96fe.tar.bz2 | |
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')
| -rw-r--r-- | lib/is_single_threaded.c | 10 |
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); 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; } |