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Date: Tue, 20 Dec 2011 13:29:05 -0800 From: Mandeep Singh Baines <msb@...omium.org> To: Frederic Weisbecker <fweisbec@...il.com> Cc: Tejun Heo <tj@...nel.org>, Li Zefan <lizf@...fujitsu.com>, LKML <linux-kernel@...r.kernel.org>, Containers <containers@...ts.linux-foundation.org>, Cgroups <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>, Mandeep Singh Baines <msb@...omium.org> Subject: Re: [PATCH 1/2] cgroup: Remove unnecessary task_lock before fetching css_set on migration Frederic Weisbecker (fweisbec@...il.com) wrote: > When we fetch the css_set of the tasks on cgroup migration, we don't need > anymore to synchronize against cgroup_exit() that could swap the old one > with init_css_set. Now that we are using threadgroup_lock() during > the migrations, we don't need to worry about it anymore. > > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com> > Cc: Tejun Heo <tj@...nel.org> > Cc: Li Zefan <lizf@...fujitsu.com> > Cc: Containers <containers@...ts.linux-foundation.org> > Cc: Cgroups <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> > Cc: Mandeep Singh Baines <msb@...omium.org> Reviewed-by: Mandeep Singh Baines <msb@...omium.org> > --- > kernel/cgroup.c | 21 +++++++++++---------- > 1 files changed, 11 insertions(+), 10 deletions(-) > > diff --git a/kernel/cgroup.c b/kernel/cgroup.c > index 5fdd98c..6b5f693 100644 > --- a/kernel/cgroup.c > +++ b/kernel/cgroup.c > @@ -1850,13 +1850,13 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp, > struct css_set *newcg; > > /* > - * get old css_set. we need to take task_lock and refcount it, because > - * an exiting task can change its css_set to init_css_set and drop its > - * old one without taking cgroup_mutex. > + * get old css_set. We are synchronized through threadgroup_lock() > + * against PF_EXITING setting such that we can't race against > + * cgroup_exit() changing the css_set to init_css_set and dropping the > + * old one. > */ > - task_lock(tsk); > + WARN_ON_ONCE(tsk->flags & PF_EXITING); > oldcg = tsk->cgroups; > - task_unlock(tsk); > > /* locate or allocate a new css_set for this task. */ > if (guarantee) { > @@ -1875,9 +1875,7 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp, > return -ENOMEM; > } > > - /* @tsk can't exit as its threadgroup is locked */ > task_lock(tsk); > - WARN_ON_ONCE(tsk->flags & PF_EXITING); > rcu_assign_pointer(tsk->cgroups, newcg); > task_unlock(tsk); > > @@ -2177,10 +2175,13 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader) > /* nothing to do if this task is already in the cgroup */ > if (tc->cgrp == cgrp) > continue; > - /* get old css_set pointer */ > - task_lock(tc->task); > + /* > + * get old css_set pointer. threadgroup is locked so this is > + * safe against concurrent cgroup_exit() changing this to > + * init_css_set. > + */ > oldcg = tc->task->cgroups; > - task_unlock(tc->task); > + > /* if we don't already have it in the list get a new one */ > if (!css_set_fetched(cgrp, tc->task, oldcg, &newcg_list)) > if (css_set_prefetch(cgrp, oldcg, &newcg_list)) > -- > 1.7.5.4 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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