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cgroup: replace tasklist_lock with rcu_read_lock
We can replace the tasklist_lock in cgroup_attach_proc with an rcu_read_lock(). Changes in V4: * https://lkml.org/lkml/2011/12/23/284 (Frederic Weisbecker) * Minimize size of rcu_read_lock critical section * Add comment * https://lkml.org/lkml/2011/12/26/136 (Li Zefan) * Split into two patches 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@gmail.com> Signed-off-by: Mandeep Singh Baines <msb@chromium.org> Acked-by: Li Zefan <lizf@cn.fujitsu.com> Acked-by: Frederic Weisbecker <fweisbec@gmail.com> Signed-off-by: Tejun Heo <tj@kernel.org> 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>
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@ -2102,10 +2102,14 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
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if (retval)
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goto out_free_group_list;
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/* prevent changes to the threadgroup list while we take a snapshot. */
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read_lock(&tasklist_lock);
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tsk = leader;
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i = 0;
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/*
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* Prevent freeing of tasks while we take a snapshot. Tasks that are
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* already PF_EXITING could be freed from underneath us unless we
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* take an rcu_read_lock.
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*/
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rcu_read_lock();
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do {
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struct task_and_cgroup ent;
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@ -2128,11 +2132,11 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
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BUG_ON(retval != 0);
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i++;
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} while_each_thread(leader, tsk);
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rcu_read_unlock();
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/* remember the number of threads in the array for later. */
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group_size = i;
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tset.tc_array = group;
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tset.tc_array_len = group_size;
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read_unlock(&tasklist_lock);
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/* methods shouldn't be called if no task is actually migrating */
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retval = 0;
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