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cgroup: pids: fix race between cgroup_post_fork() and cgroup_migrate()

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If the new child migrates to another cgroup before cgroup_post_fork() calls
subsys->fork(), then both pids_can_attach() and pids_fork() will do the same
pids_uncharge(old_pids) + pids_charge(pids) sequence twice.

Change copy_process() to call threadgroup_change_begin/threadgroup_change_end
unconditionally. percpu_down_read() is cheap and this allows other cleanups,
see the next changes.

Also, this way we can unify cgroup_threadgroup_rwsem and dup_mmap_sem.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Oleg Nesterov
2015-11-27 19:57:19 +01:00
committed by Tejun Heo
parent 53254f900b
commit c9e75f0492
2 changed files with 5 additions and 25 deletions
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9
kernel/fork.c
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@@ -1368,8 +1368,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
p->real_start_time = ktime_get_boot_ns();
p->io_context = NULL;
p->audit_context = NULL;
if (clone_flags & CLONE_THREAD)
threadgroup_change_begin(current);
threadgroup_change_begin(current);
cgroup_fork(p);
#ifdef CONFIG_NUMA
p->mempolicy = mpol_dup(p->mempolicy);
@@ -1610,8 +1609,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
proc_fork_connector(p);
cgroup_post_fork(p, cgrp_ss_priv);
if (clone_flags & CLONE_THREAD)
threadgroup_change_end(current);
threadgroup_change_end(current);
perf_event_fork(p);
trace_task_newtask(p, clone_flags);
@@ -1652,8 +1650,7 @@ bad_fork_cleanup_policy:
mpol_put(p->mempolicy);
bad_fork_cleanup_threadgroup_lock:
#endif
if (clone_flags & CLONE_THREAD)
threadgroup_change_end(current);
threadgroup_change_end(current);
delayacct_tsk_free(p);
bad_fork_cleanup_count:
atomic_dec(&p->cred->user->processes);