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Message-Id: <1325654312-32477-1-git-send-email-msb@chromium.org>
Date: Tue, 3 Jan 2012 21:18:30 -0800
From: Mandeep Singh Baines <msb@...omium.org>
To: Tejun Heo <tj@...nel.org>, Li Zefan <lizf@...fujitsu.com>,
Frederic Weisbecker <fweisbec@...il.com>,
linux-kernel@...r.kernel.org
Cc: Mandeep Singh Baines <msb@...omium.org>, Tejun Heo <tj@...nel.org>,
Li Zefan <lizf@...fujitsu.com>,
Frederic Weisbecker <fweisbec@...il.com>,
containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Oleg Nesterov <oleg@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Paul Menage <paul@...lmenage.org>
Subject: [PATCH 1/3] cgroup: simplify double-check locking in cgroup_attach_proc
To keep the complexity of the double-check locking in one place,
I moved the thread_group_leader check up into attach_task_by_pid.
This allows us to use a goto instead of returning -EAGAIN.
While at it, converted a couple of returns to gotos and used
rcu for the !pid case also in order to simplify the logic.
Changes in V2:
* https://lkml.org/lkml/2011/12/22/86 (Tejun Heo)
* Use a goto instead of returning -EAGAIN
Signed-off-by: Mandeep Singh Baines <msb@...omium.org>
Cc: Tejun Heo <tj@...nel.org>
Cc: Li Zefan <lizf@...fujitsu.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: containers@...ts.linux-foundation.org
Cc: cgroups@...r.kernel.org
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Paul Menage <paul@...lmenage.org>
---
kernel/cgroup.c | 79 ++++++++++++++++++++----------------------------------
1 files changed, 29 insertions(+), 50 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1042b3c..09867eb 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2104,19 +2104,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
/* 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;
- goto out_free_group_list;
- }
-
tsk = leader;
i = 0;
do {
@@ -2242,22 +2229,14 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
if (!cgroup_lock_live_group(cgrp))
return -ENODEV;
+retry_find_task:
+ rcu_read_lock();
if (pid) {
- rcu_read_lock();
tsk = find_task_by_vpid(pid);
if (!tsk) {
rcu_read_unlock();
- cgroup_unlock();
- return -ESRCH;
- }
- if (threadgroup) {
- /*
- * RCU protects this access, since tsk was found in the
- * tid map. a race with de_thread may cause group_leader
- * to stop being the leader, but cgroup_attach_proc will
- * detect it later.
- */
- tsk = tsk->group_leader;
+ ret= -ESRCH;
+ goto out_unlock_cgroup;
}
/*
* even if we're attaching all tasks in the thread group, we
@@ -2268,29 +2247,38 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
cred->euid != tcred->uid &&
cred->euid != tcred->suid) {
rcu_read_unlock();
- cgroup_unlock();
- return -EACCES;
+ ret = -EACCES;
+ goto out_unlock_cgroup;
}
- get_task_struct(tsk);
- rcu_read_unlock();
- } else {
- if (threadgroup)
- tsk = current->group_leader;
- else
- tsk = current;
- get_task_struct(tsk);
- }
-
- threadgroup_lock(tsk);
+ } else
+ tsk = current;
if (threadgroup)
+ tsk = tsk->group_leader;
+ get_task_struct(tsk);
+ rcu_read_unlock();
+
+ threadgroup_lock(tsk);
+ if (threadgroup) {
+ if (!thread_group_leader(tsk)) {
+ /*
+ * a race with de_thread from another thread's exec()
+ * may strip us of our leadership, if this happens,
+ * there is no choice but to throw this task away and
+ * try again; this is
+ * "double-double-toil-and-trouble-check locking".
+ */
+ threadgroup_unlock(tsk);
+ put_task_struct(tsk);
+ goto retry_find_task;
+ }
ret = cgroup_attach_proc(cgrp, tsk);
- else
+ } else
ret = cgroup_attach_task(cgrp, tsk);
-
threadgroup_unlock(tsk);
put_task_struct(tsk);
+out_unlock_cgroup:
cgroup_unlock();
return ret;
}
@@ -2302,16 +2290,7 @@ static int cgroup_tasks_write(struct cgroup *cgrp, struct cftype *cft, u64 pid)
static int cgroup_procs_write(struct cgroup *cgrp, struct cftype *cft, u64 tgid)
{
- int ret;
- do {
- /*
- * attach_proc fails with -EAGAIN if threadgroup leadership
- * changes in the middle of the operation, in which case we need
- * to find the task_struct for the new leader and start over.
- */
- ret = attach_task_by_pid(cgrp, tgid, true);
- } while (ret == -EAGAIN);
- return ret;
+ return attach_task_by_pid(cgrp, tgid, true);
}
/**
--
1.7.3.1
--
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