[PATCH AUTOSEL 4.14 095/123] cgroup/pids: turn cgroup_subsys->free() into cgroup_subsys->release() to fix the accounting

Sasha Levin
2019-03-27T18:47:42Z

From: Oleg Nesterov <oleg@redhat.com>

[ Upstream commit 51bee5abeab2058ea5813c5615d6197a23dbf041 ]

The only user of cgroup_subsys->free() callback is pids_cgrp_subsys which
needs pids_free() to uncharge the pid.

However, ->free() is called from __put_task_struct()->cgroup_free() and this
is too late. Even the trivial program which does

for (;;) {
int pid = fork();
assert(pid >= 0);
if (pid)
wait(NULL);
else
exit(0);
}

can run out of limits because release_task()->call_rcu(delayed_put_task_struct)
implies an RCU gp after the task/pid goes away and before the final put().

Test-case:

mkdir -p /tmp/CG
mount -t cgroup2 none /tmp/CG
echo '+pids' > /tmp/CG/cgroup.subtree_control

mkdir /tmp/CG/PID
echo 2 > /tmp/CG/PID/pids.max

perl -e 'while ($p = fork) { wait; } $p // die "fork failed: $!\n"' &
echo $! > /tmp/CG/PID/cgroup.procs

Without this patch the forking process fails soon after migration.

Rename cgroup_subsys->free() to cgroup_subsys->release() and move the callsite
into the new helper, cgroup_release(), called by release_task() which actually
frees the pid(s).

Reported-by: Herton R. Krzesinski <hkrzesin@redhat.com>
Reported-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/cgroup-defs.h | 2 +-
include/linux/cgroup.h | 2 ++
kernel/cgroup/cgroup.c | 15 +++++++++------
kernel/cgroup/pids.c | 4 ++--
kernel/exit.c | 1 +
5 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index e7905d9353e8..93a2469a9130 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -523,7 +523,7 @@ struct cgroup_subsys {
void (*cancel_fork)(struct task_struct *task);
void (*fork)(struct task_struct *task);
void (*exit)(struct task_struct *task);
- void (*free)(struct task_struct *task);
+ void (*release)(struct task_struct *task);
void (*bind)(struct cgroup_subsys_state *root_css);

bool early_init:1;
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index dddbc29e2009..8e83c9055ccb 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -118,6 +118,7 @@ extern int cgroup_can_fork(struct task_struct *p);
extern void cgroup_cancel_fork(struct task_struct *p);
extern void cgroup_post_fork(struct task_struct *p);
void cgroup_exit(struct task_struct *p);
+void cgroup_release(struct task_struct *p);
void cgroup_free(struct task_struct *p);

int cgroup_init_early(void);
@@ -668,6 +669,7 @@ static inline int cgroup_can_fork(struct task_struct *p) { return 0; }
static inline void cgroup_cancel_fork(struct task_struct *p) {}
static inline void cgroup_post_fork(struct task_struct *p) {}
static inline void cgroup_exit(struct task_struct *p) {}
+static inline void cgroup_release(struct task_struct *p) {}
static inline void cgroup_free(struct task_struct *p) {}

static inline int cgroup_init_early(void) { return 0; }
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 7e79358b4473..694b1cc8d144 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -187,7 +187,7 @@ static u64 css_serial_nr_next = 1;
*/
static u16 have_fork_callback __read_mostly;
static u16 have_exit_callback __read_mostly;
-static u16 have_free_callback __read_mostly;
+static u16 have_release_callback __read_mostly;
static u16 have_canfork_callback __read_mostly;

/* cgroup namespace for init task */
@@ -5112,7 +5112,7 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)

have_fork_callback |= (bool)ss->fork << ss->id;
have_exit_callback |= (bool)ss->exit << ss->id;
- have_free_callback |= (bool)ss->free << ss->id;
+ have_release_callback |= (bool)ss->release << ss->id;
have_canfork_callback |= (bool)ss->can_fork << ss->id;

/* At system boot, before all subsystems have been
@@ -5546,16 +5546,19 @@ void cgroup_exit(struct task_struct *tsk)
} while_each_subsys_mask();
}

-void cgroup_free(struct task_struct *task)
+void cgroup_release(struct task_struct *task)
{
- struct css_set *cset = task_css_set(task);
struct cgroup_subsys *ss;
int ssid;

- do_each_subsys_mask(ss, ssid, have_free_callback) {
- ss->free(task);
+ do_each_subsys_mask(ss, ssid, have_release_callback) {
+ ss->release(task);
} while_each_subsys_mask();
+}

+void cgroup_free(struct task_struct *task)
+{
+ struct css_set *cset = task_css_set(task);
put_css_set(cset);
}

diff --git a/kernel/cgroup/pids.c b/kernel/cgroup/pids.c
index 9829c67ebc0a..c9960baaa14f 100644
--- a/kernel/cgroup/pids.c
+++ b/kernel/cgroup/pids.c
@@ -247,7 +247,7 @@ static void pids_cancel_fork(struct task_struct *task)
pids_uncharge(pids, 1);
}

-static void pids_free(struct task_struct *task)
+static void pids_release(struct task_struct *task)
{
struct pids_cgroup *pids = css_pids(task_css(task, pids_cgrp_id));

@@ -342,7 +342,7 @@ struct cgroup_subsys pids_cgrp_subsys = {
.cancel_attach = pids_cancel_attach,
.can_fork = pids_can_fork,
.cancel_fork = pids_cancel_fork,
- .free = pids_free,
+ .release = pids_release,
.legacy_cftypes = pids_files,
.dfl_cftypes = pids_files,
.threaded = true,
diff --git a/kernel/exit.c b/kernel/exit.c
index 5523fb0c20c8..95ce231ff5e2 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -218,6 +218,7 @@ void release_task(struct task_struct *p)
}

write_unlock_irq(&tasklist_lock);
+ cgroup_release(p);
release_thread(p);
call_rcu(&p->rcu, delayed_put_task_struct);

--
2.19.1