cgroups: more safe tasklist locking in cgroup_attach_proc
Fix unstable tasklist locking in cgroup_attach_proc. According to this thread - https://lkml.org/lkml/2011/7/27/243 - RCU is not sufficient to guarantee the tasklist is stable w.r.t. de_thread and exit. Taking tasklist_lock for reading, instead of rcu_read_lock, ensures proper exclusion. Signed-off-by: Ben Blum <bblum@andrew.cmu.edu> Acked-by: Paul Menage <paul@paulmenage.org> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Cc: Neil Brown <neilb@suse.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -2027,7 +2027,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
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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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rcu_read_lock();
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read_lock(&tasklist_lock);
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if (!thread_group_leader(leader)) {
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/*
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* a race with de_thread from another thread's exec() may strip
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@ -2036,7 +2036,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
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* throw this task away and try again (from cgroup_procs_write);
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* this is "double-double-toil-and-trouble-check locking".
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*/
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rcu_read_unlock();
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read_unlock(&tasklist_lock);
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retval = -EAGAIN;
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goto out_free_group_list;
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}
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@ -2057,7 +2057,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
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} while_each_thread(leader, tsk);
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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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rcu_read_unlock();
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read_unlock(&tasklist_lock);
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/*
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* step 1: check that we can legitimately attach to the cgroup.
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