linux-kernel - [PATCH v2 -mm 2/2] current_is_single_threaded: don't use ->mmap

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Message-ID: <20090710014857.GC4723@redhat.com>
Date:	Fri, 10 Jul 2009 03:48:57 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	James Morris <jmorris@...ei.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...hat.com>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 -mm 2/2] current_is_single_threaded: don't use ->mmap_sem

current_is_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@...hat.com>
---

 lib/is_single_threaded.c |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

--- WAIT/lib/is_single_threaded.c~ISS_2_RCU	2009-07-09 22:16:29.000000000 +0200
+++ WAIT/lib/is_single_threaded.c	2009-07-09 22:54:41.000000000 +0200
@@ -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;
 }

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