Since cgroup_attach_proc is protected by a threadgroup_lock, we can replace the tasklist_lock in cgroup_attach_proc with an rcu_read_lock. To keep the complexity of the double-check locking in one place, I also 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, also converted a couple of returns to gotos. Changes in V3: * https://lkml.org/lkml/2011/12/22/419 (Frederic Weisbecker) * Add an rcu_read_lock to protect against exit Changes in V2: * https://lkml.org/lkml/2011/12/22/86 (Tejun Heo) * Use a goto instead of returning -EAGAIN Suggested-by: Frederic Weisbecker <fweisbec@xxxxxxxxx> Signed-off-by: Mandeep Singh Baines <msb@xxxxxxxxxxxx> Cc: Tejun Heo <tj@xxxxxxxxxx> Cc: Li Zefan <lizf@xxxxxxxxxxxxxx> Cc: containers@xxxxxxxxxxxxxxxxxxxxxxxxxx Cc: cgroups@xxxxxxxxxxxxxxx Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> Cc: Oleg Nesterov <oleg@xxxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Paul Menage <paul@xxxxxxxxxxxxxx> --- kernel/cgroup.c | 74 +++++++++++++++++++----------------------------------- 1 files changed, 26 insertions(+), 48 deletions(-) diff --git a/kernel/cgroup.c b/kernel/cgroup.c index 1042b3c..6ee1438 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -2102,21 +2102,7 @@ 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; - goto out_free_group_list; - } - + rcu_read_lock(); tsk = leader; i = 0; do { @@ -2145,7 +2131,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader) group_size = i; tset.tc_array = group; tset.tc_array_len = group_size; - read_unlock(&tasklist_lock); + rcu_read_unlock(); /* methods shouldn't be called if no task is actually migrating */ retval = 0; @@ -2242,22 +2228,14 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup) if (!cgroup_lock_live_group(cgrp)) return -ENODEV; +retry_find_task: 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 +2246,38 @@ 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; + tsk = current; get_task_struct(tsk); } threadgroup_lock(tsk); - - if (threadgroup) - ret = cgroup_attach_proc(cgrp, tsk); - else + if (threadgroup) { + struct task_struct *leader = tsk->group_leader; + if (!thread_group_leader(leader)) { + /* + * 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, leader); + } else ret = cgroup_attach_task(cgrp, tsk); - threadgroup_unlock(tsk); put_task_struct(tsk); +out_unlock_cgroup: cgroup_unlock(); return ret; } @@ -2302,16 +2289,7 @@ 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 _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
