linux-kernel - [PATCH 1/2] cgroups: use sighand lock instead of tasklist_lock in cgroup_attach

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Date:	Thu, 13 Oct 2011 20:31:15 -0400
From:	Ben Blum <bblum@...rew.cmu.edu>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org
Cc:	Ben Blum <bblum@...rew.cmu.edu>, NeilBrown <neilb@...e.de>,
	paulmck@...ux.vnet.ibm.com, Paul Menage <paul@...lmenage.org>,
	Li Zefan <lizf@...fujitsu.com>,
	containers@...ts.linux-foundation.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Balbir Singh <bsingharora@...il.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: [PATCH 1/2] cgroups: use sighand lock instead of tasklist_lock in
 cgroup_attach_proc

Use sighand lock instead of tasklist_lock in cgroup_attach_proc.

From: Ben Blum <bblum@...rew.cmu.edu>

Signed-off-by: Ben Blum <bblum@...rew.cmu.edu>
---
 kernel/cgroup.c |   27 +++++++++++++--------------
 1 files changed, 13 insertions(+), 14 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 893fc3d..32fb4c8 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2003,6 +2003,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 	/* threadgroup list cursor and array */
 	struct task_struct *tsk;
 	struct flex_array *group;
+	unsigned long flags;
 	/*
 	 * we need to make sure we have css_sets for all the tasks we're
 	 * going to move -before- we actually start moving them, so that in
@@ -2029,20 +2030,18 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 	if (retval)
 		goto out_free_group_list;
 
-	/* prevent changes to the threadgroup list while we take a snapshot. */
-	read_lock(&tasklist_lock);
-	if (!thread_group_leader(leader)) {
-		/*
-		 * a race with de_thread from another thread's exec() may strip
-		 * us of our leadership, making while_each_thread unsafe to use
-		 * on this task. if this happens, there is no choice but to
-		 * throw this task away and try again (from cgroup_procs_write);
-		 * this is "double-double-toil-and-trouble-check locking".
-		 */
-		read_unlock(&tasklist_lock);
-		retval = -EAGAIN;
+	/* prevent changes to the threadgroup list while we take a snapshot.
+	 * If the leader exits, its links on the thread_group list become
+	 * invalid. One way this can happen is if a sub-thread does exec() when
+	 * de_thread() calls release_task(leader) (and leader->sighand gets set
+	 * to NULL, in which case lock_task_sighand will fail). Since in that
+	 * case the threadgroup is still around, cgroup_procs_write should try
+	 * again (finding the new leader), which EAGAIN indicates here. This is
+	 * double-double-toil-and-trouble-check locking". */
+	retval = -EAGAIN;
+	if (!lock_task_sighand(leader, &flags))
 		goto out_free_group_list;
-	}
+
 	/* take a reference on each task in the group to go in the array. */
 	tsk = leader;
 	i = 0;
@@ -2058,9 +2057,9 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 		BUG_ON(retval != 0);
 		i++;
 	} while_each_thread(leader, tsk);
+	unlock_task_sighand(leader, &flags);
 	/* remember the number of threads in the array for later. */
 	group_size = i;
-	read_unlock(&tasklist_lock);
 
 	/*
 	 * step 1: check that we can legitimately attach to the cgroup.
--
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