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Subject[PATCH 1/3] cgroup: pids: fix race between cgroup_post_fork() and cgroup_migrate()
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>
---
kernel/cgroup_pids.c | 21 ++-------------------
kernel/fork.c | 9 +++------
2 files changed, 5 insertions(+), 25 deletions(-)

diff --git a/kernel/cgroup_pids.c b/kernel/cgroup_pids.c
index cdd8df4..15ef2e4 100644
--- a/kernel/cgroup_pids.c
+++ b/kernel/cgroup_pids.c
@@ -243,27 +243,10 @@ static void pids_cancel_fork(struct task_struct *task, void *priv)

static void pids_fork(struct task_struct *task, void *priv)
{
- struct cgroup_subsys_state *css;
- struct cgroup_subsys_state *old_css = priv;
- struct pids_cgroup *pids;
- struct pids_cgroup *old_pids = css_pids(old_css);
-
- css = task_get_css(task, pids_cgrp_id);
- pids = css_pids(css);
-
- /*
- * If the association has changed, we have to revert and reapply the
- * charge/uncharge on the wrong hierarchy to the current one. Since
- * the association can only change due to an organisation event, its
- * okay for us to ignore the limit in this case.
- */
- if (pids != old_pids) {
- pids_uncharge(old_pids, 1);
- pids_charge(pids, 1);
- }
+ struct cgroup_subsys_state *css = priv;

+ WARN_ON(task_css_check(task, pids_cgrp_id, true) != css);
css_put(css);
- css_put(old_css);
}

static void pids_free(struct task_struct *task)
diff --git a/kernel/fork.c b/kernel/fork.c
index f97f2c4..fce002e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -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);
--
1.5.5.1


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