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Subject[RFC v17][PATCH 28/60] c/r: support for zombie processes
Date
During checkpoint, a zombie processes need only save p->comm,
p->state, p->exit_state, and p->exit_code.

During restart, zombie processes are created like all other
processes. They validate the saved exit_code restore p->comm
and p->exit_code. Then they call do_exit() instead of waking
up the next task in line.

But before, they place the @ctx in p->checkpoint_ctx, so that
only at exit time they will wake up the next task in line,
and drop the reference to the @ctx.

This provides the guarantee that when the coordinator's wait
completes, all normal tasks completed their restart, and all
zombie tasks are already zombified (as opposed to perhap only
becoming a zombie).

Changelog[v17]:
- Validate t->exit_signal for both threads and leader
- Skip zombies in most of may_checkpoint_task()
- Save/restore t->pdeath_signal
- Validate ->exit_signal and ->pdeath_signal

Signed-off-by: Oren Laadan <orenl@cs.columbia.edu>
---
checkpoint/checkpoint.c | 12 +++++--
checkpoint/process.c | 67 +++++++++++++++++++++++++++++++++++-----
checkpoint/restart.c | 40 +++++++++++++++++++++---
include/linux/checkpoint.h | 1 +
include/linux/checkpoint_hdr.h | 1 +
5 files changed, 104 insertions(+), 17 deletions(-)

diff --git a/checkpoint/checkpoint.c b/checkpoint/checkpoint.c
index 57f59de..fb14585 100644
--- a/checkpoint/checkpoint.c
+++ b/checkpoint/checkpoint.c
@@ -280,8 +280,8 @@ static int may_checkpoint_task(struct ckpt_ctx *ctx, struct task_struct *t)

ckpt_debug("check %d\n", task_pid_nr_ns(t, ctx->root_nsproxy->pid_ns));

- if (t->state == TASK_DEAD) {
- pr_warning("c/r: task %d is TASK_DEAD\n", task_pid_vnr(t));
+ if (t->exit_state == EXIT_DEAD) {
+ pr_warning("c/r: task %d is EXIT_DEAD\n", task_pid_vnr(t));
return -EAGAIN;
}

@@ -291,6 +291,10 @@ static int may_checkpoint_task(struct ckpt_ctx *ctx, struct task_struct *t)
return -EPERM;
}

+ /* zombies are cool (and also don't have nsproxy, below...) */
+ if (t->exit_state)
+ return 0;
+
/* verify that all tasks belongs to same freezer cgroup */
if (t != current && !in_same_cgroup_freezer(t, ctx->root_freezer)) {
__ckpt_write_err(ctx, "task %d (%s) not frozen (wrong cgroup)",
@@ -309,8 +313,8 @@ static int may_checkpoint_task(struct ckpt_ctx *ctx, struct task_struct *t)
* FIX: for now, disallow siblings of container init created
* via CLONE_PARENT (unclear if they will remain possible)
*/
- if (ctx->root_init && t != root && t->tgid != root->tgid &&
- t->real_parent == root->real_parent) {
+ if (ctx->root_init && t != root &&
+ t->real_parent == root->real_parent && t->tgid != root->tgid) {
__ckpt_write_err(ctx, "task %d (%s) is sibling of root",
task_pid_vnr(t), t->comm);
return -EINVAL;
diff --git a/checkpoint/process.c b/checkpoint/process.c
index a0bf344..a67c389 100644
--- a/checkpoint/process.c
+++ b/checkpoint/process.c
@@ -35,12 +35,18 @@ static int checkpoint_task_struct(struct ckpt_ctx *ctx, struct task_struct *t)
h->state = t->state;
h->exit_state = t->exit_state;
h->exit_code = t->exit_code;
- h->exit_signal = t->exit_signal;

- h->set_child_tid = t->set_child_tid;
- h->clear_child_tid = t->clear_child_tid;
+ if (t->exit_state) {
+ /* zombie - skip remaining state */
+ BUG_ON(t->exit_state != EXIT_ZOMBIE);
+ } else {
+ /* FIXME: save remaining relevant task_struct fields */
+ h->exit_signal = t->exit_signal;
+ h->pdeath_signal = t->pdeath_signal;

- /* FIXME: save remaining relevant task_struct fields */
+ h->set_child_tid = t->set_child_tid;
+ h->clear_child_tid = t->clear_child_tid;
+ }

ret = ckpt_write_obj(ctx, &h->h);
ckpt_hdr_put(ctx, h);
@@ -169,6 +175,11 @@ int checkpoint_task(struct ckpt_ctx *ctx, struct task_struct *t)
ckpt_debug("task %d\n", ret);
if (ret < 0)
goto out;
+
+ /* zombie - we're done here */
+ if (t->exit_state)
+ return 0;
+
ret = checkpoint_thread(ctx, t);
ckpt_debug("thread %d\n", ret);
if (ret < 0)
@@ -187,6 +198,19 @@ int checkpoint_task(struct ckpt_ctx *ctx, struct task_struct *t)
* Restart
*/

+static inline int valid_exit_code(int exit_code)
+{
+ if (exit_code >= 0x10000)
+ return 0;
+ if (exit_code & 0xff) {
+ if (exit_code & ~0xff)
+ return 0;
+ if (!valid_signal(exit_code & 0xff))
+ return 0;
+ }
+ return 1;
+}
+
/* read the task_struct into the current task */
static int restore_task_struct(struct ckpt_ctx *ctx)
{
@@ -198,15 +222,37 @@ static int restore_task_struct(struct ckpt_ctx *ctx)
if (IS_ERR(h))
return PTR_ERR(h);

+ ret = -EINVAL;
+ if (h->state == TASK_DEAD) {
+ if (h->exit_state != EXIT_ZOMBIE)
+ goto out;
+ if (!valid_exit_code(h->exit_code))
+ goto out;
+ t->exit_code = h->exit_code;
+ } else {
+ if (h->exit_code)
+ goto out;
+ if ((thread_group_leader(t) && !valid_signal(h->exit_signal)) ||
+ (!thread_group_leader(t) && h->exit_signal != -1))
+ goto out;
+ if (!valid_signal(h->pdeath_signal))
+ goto out;
+
+ /* FIXME: restore remaining relevant task_struct fields */
+ t->exit_signal = h->exit_signal;
+ t->pdeath_signal = h->pdeath_signal;
+
+ t->set_child_tid = h->set_child_tid;
+ t->clear_child_tid = h->clear_child_tid;
+ }
+
memset(t->comm, 0, TASK_COMM_LEN);
ret = _ckpt_read_string(ctx, t->comm, TASK_COMM_LEN);
if (ret < 0)
goto out;

- t->set_child_tid = h->set_child_tid;
- t->clear_child_tid = h->clear_child_tid;
-
- /* FIXME: restore remaining relevant task_struct fields */
+ /* return 1 for zombie, 0 otherwise */
+ ret = (h->state == TASK_DEAD ? 1 : 0);
out:
ckpt_hdr_put(ctx, h);
return ret;
@@ -326,6 +372,11 @@ int restore_task(struct ckpt_ctx *ctx)
ckpt_debug("task %d\n", ret);
if (ret < 0)
goto out;
+
+ /* zombie - we're done here */
+ if (ret)
+ goto out;
+
ret = restore_thread(ctx);
ckpt_debug("thread %d\n", ret);
if (ret < 0)
diff --git a/checkpoint/restart.c b/checkpoint/restart.c
index 65422e2..1b1f639 100644
--- a/checkpoint/restart.c
+++ b/checkpoint/restart.c
@@ -375,20 +375,17 @@ static inline void ckpt_notify_error(struct ckpt_ctx *ctx)
complete(&ctx->complete);
}

-static int ckpt_activate_next(struct ckpt_ctx *ctx)
+int ckpt_activate_next(struct ckpt_ctx *ctx)
{
struct task_struct *task;
- int active;
pid_t pid;

- active = ++ctx->active_pid;
- if (active >= ctx->nr_pids) {
+ if (++ctx->active_pid >= ctx->nr_pids) {
complete(&ctx->complete);
return 0;
}

pid = get_active_pid(ctx);
- ckpt_debug("active pid %d (%d < %d)\n", pid, active, ctx->nr_pids);

rcu_read_lock();
task = find_task_by_pid_ns(pid, ctx->root_nsproxy->pid_ns);
@@ -413,6 +410,8 @@ static int wait_task_active(struct ckpt_ctx *ctx)
ret = wait_event_interruptible(ctx->waitq,
is_task_active(ctx, pid) ||
ckpt_test_ctx_error(ctx));
+ ckpt_debug("active %d < %d (ret %d)\n",
+ ctx->active_pid, ctx->nr_pids, ret);
if (!ret && ckpt_test_ctx_error(ctx)) {
force_sig(SIGKILL, current);
ret = -EBUSY;
@@ -468,6 +467,8 @@ static int do_restore_task(void)
return -EAGAIN;
}

+ current->flags |= PF_RESTARTING;
+
/* wait for our turn, do the restore, and tell next task in line */
ret = wait_task_active(ctx);
if (ret < 0)
@@ -477,6 +478,13 @@ static int do_restore_task(void)
if (ret < 0)
goto out;

+ /*
+ * zombie: we're done here; Save @ctx on task_struct, to be
+ * used to ckpt_activate_next(), and released, from do_exit().
+ */
+ if (ret)
+ do_exit(current->exit_code);
+
ret = ckpt_activate_next(ctx);
if (ret < 0)
goto out;
@@ -493,6 +501,7 @@ static int do_restore_task(void)
wake_up_all(&ctx->waitq);
}

+ current->flags &= ~PF_RESTARTING;
ckpt_ctx_put(ctx);
return ret;
}
@@ -593,6 +602,7 @@ static int wait_all_tasks_finish(struct ckpt_ctx *ctx)

ret = wait_for_completion_interruptible(&ctx->complete);

+ ckpt_debug("final sync kflags %#lx\n", ctx->kflags);
if (ckpt_test_ctx_error(ctx))
ret = -EBUSY;
return ret;
@@ -820,3 +830,23 @@ long do_restart(struct ckpt_ctx *ctx, pid_t pid)

return ret;
}
+
+/**
+ * exit_checkpoint - callback from do_exit to cleanup checkpoint state
+ * @tsk: terminating task
+ */
+void exit_checkpoint(struct task_struct *tsk)
+{
+ struct ckpt_ctx *ctx;
+
+ ctx = tsk->checkpoint_ctx;
+ tsk->checkpoint_ctx = NULL;
+
+ /* restarting zombies will acrivate next task in restart */
+ if (tsk->flags & PF_RESTARTING) {
+ if (ckpt_activate_next(ctx) < 0)
+ pr_warning("c/r: [%d] failed zombie exit\n", tsk->pid);
+ }
+
+ ckpt_ctx_put(ctx);
+}
diff --git a/include/linux/checkpoint.h b/include/linux/checkpoint.h
index 44b692d..b6af5b9 100644
--- a/include/linux/checkpoint.h
+++ b/include/linux/checkpoint.h
@@ -85,6 +85,7 @@ extern long do_checkpoint(struct ckpt_ctx *ctx, pid_t pid);
extern long do_restart(struct ckpt_ctx *ctx, pid_t pid);

/* task */
+extern int ckpt_activate_next(struct ckpt_ctx *ctx);
extern int checkpoint_task(struct ckpt_ctx *ctx, struct task_struct *t);
extern int restore_task(struct ckpt_ctx *ctx);

diff --git a/include/linux/checkpoint_hdr.h b/include/linux/checkpoint_hdr.h
index c9a80dc..3f2db22 100644
--- a/include/linux/checkpoint_hdr.h
+++ b/include/linux/checkpoint_hdr.h
@@ -130,6 +130,7 @@ struct ckpt_hdr_task {
__u32 exit_state;
__u32 exit_code;
__u32 exit_signal;
+ __u32 pdeath_signal;

__u64 set_child_tid;
__u64 clear_child_tid;
--
1.6.0.4


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