Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

On Thu, Oct 05, 2017 at 04:02:06PM +0000, Mathieu Desnoyers wrote:

----- On Oct 5, 2017, at 8:12 AM, Peter Zijlstra peterz@infradead.org wrote:

quoted

On Wed, Oct 04, 2017 at 02:37:53PM -0700, Paul E. McKenney wrote:

quoted

diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c
new file mode 100644
index 000000000000..b0d79a5f5981
--- /dev/null
+++ b/arch/powerpc/kernel/membarrier.c

@@ -0,0 +1,45 @@

quoted

+void membarrier_arch_register_private_expedited(struct task_struct *p)
+{
+	struct task_struct *t;
+
+	if (get_nr_threads(p) == 1) {
+		set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
+		return;
+	}
+	/*
+	 * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
+	 * fork is protected by siglock.
+	 */
+	spin_lock(&p->sighand->siglock);
+	for_each_thread(p, t)
+		set_ti_thread_flag(task_thread_info(t),
+				TIF_MEMBARRIER_PRIVATE_EXPEDITED);

I'm not sure this works correctly vs CLONE_VM without CLONE_THREAD.

The intent here is to hold the sighand siglock to provide mutual
exclusion against invocation of membarrier_fork(p, clone_flags)
by copy_process().

copy_process() grabs spin_lock(&current->sighand->siglock) for both
CLONE_THREAD and not CLONE_THREAD flags.

What am I missing here ?

If you do CLONE_VM without CLONE_THREAD you'll end up sharing the mm but
you'll not be part of thread_head, so the for_each_thread() iteration
will not find the task.