Messages in this thread Patch in this message | | | Date | Sat, 22 Nov 2008 10:02:29 +0800 | From | Li Zefan <> | Subject | Re: [PATCH 1/2] cgroups: enhance task_cgroup() |
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Paul Menage wrote: > On Fri, Nov 21, 2008 at 12:49 AM, Lai Jiangshan <laijs@cn.fujitsu.com> wrote: >> task_cgroup() calls cgroup_subsys_state(). > > No, it calls task_subsys_state() > >> and we must use rcu_read_lock() to protect cgroup_subsys_state(). >> so we must use rcu_read_lock() to protect task_cgroup(). >> >> but it'll not so friendly to caller: the callers of task_cgroup() have >> held cgroup_lock(). it means that struct cgroup will not be freed. >> >> So this patch add rcu_read_lock() in task_cgroup() to enhance task_cgroup(). >> And we do NOT NEED FIX task_cgroup()'s callers, and cgroup_lock() >> can protect task_cgroup(). > > Is there a reason to add an implicit rcu_read_lock() in task_cgroup() > and not directly in task_subsys_state() ? >
I don't think it's a good idea to add rcu_read_lock() in task_cgroup() or task_subsys_state(). The returned pointer of the 2 functions should be protected by rcu_read_lock or cgroup_lock, so the caller should still hold the lock by itself.
An example of this is:
static int mem_cgroup_charge_common(...) { ... rcu_read_lock(); mem = mem_cgroup_from_task(rcu_dereference(mm->owner)); if (unlikely(!mem)) { rcu_read_unlock(); return 0; } /* * For every charge from the cgroup, increment reference count */ css_get(&mem->css); rcu_read_unlock(); ... }
> Paul > >> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com> >> --- >> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h >> index 1164963..22901ff 100644 >> --- a/include/linux/cgroup.h >> +++ b/include/linux/cgroup.h >> @@ -359,6 +360,10 @@ >> static inline struct cgroup* task_cgroup(struct task_struct *task, >> int subsys_id) >> { >> - return task_subsys_state(task, subsys_id)->cgroup; >> + struct cgroup *ret; >> + rcu_read_lock(); >> + ret = task_subsys_state(task, subsys_id)->cgroup; >> + rcu_read_unlock(); >> + return ret; >> } >> >> int cgroup_clone(struct task_struct *tsk, struct cgroup_subsys *ss, >> >> >>
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