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cgroup: simplify double-check locking in cgroup_attach_proc 

To keep the complexity of the double-check locking in one place, move
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, convert a couple of returns to gotos and use 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@chromium.org>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: containers@lists.linux-foundation.org
Cc: cgroups@vger.kernel.org
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Menage <paul@paulmenage.org>
This commit is contained in:
Mandeep Singh Baines 2012-01-03 21:18:30 -08:00 committed by Tejun Heo
parent 245282557c
commit b78949ebfb
1 changed files with 30 additions and 51 deletions
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81
kernel/cgroup.c
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@ -2104,19 +2104,6 @@ static 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 {
@ -2245,22 +2232,14 @@ 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
@ -2271,29 +2250,38 @@ 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)
ret = cgroup_attach_proc(cgrp, tsk);
else
ret = cgroup_attach_task(cgrp, tsk);
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
ret = cgroup_attach_task(cgrp, tsk);
threadgroup_unlock(tsk);
put_task_struct(tsk);
out_unlock_cgroup:
cgroup_unlock();
return ret;
}
@ -2305,16 +2293,7 @@ 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);
}
/**