Re: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression

From: Michael S. Tsirkin
Date: Mon May 22 2023 - 15:46:03 EST

On Sun, May 21, 2023 at 09:51:24PM -0500, Mike Christie wrote:
> When switching from kthreads to vhost_tasks two bugs were added:
> 1. The vhost worker tasks's now show up as processes so scripts doing ps
> or ps a would not incorrectly detect the vhost task as another process.
> 2. kthreads disabled freeze by setting PF_NOFREEZE, but vhost tasks's
> didn't disable or add support for them.
>
> To fix both bugs, this switches the vhost task to be thread in the
> process that does the VHOST_SET_OWNER ioctl, and has vhost_worker call
> get_signal to support SIGKILL/SIGSTOP and freeze signals. Note that
> SIGKILL/STOP support is required because CLONE_THREAD requires
> CLONE_SIGHAND which requires those 2 signals to be suppported.
>
> This a modified version of patch originally written by Linus which
> handles his review comment to himself to rename ignore_signals to
> block_signals to better represent what it now does. And it includes a
> change to vhost_worker() to support SIGSTOP/KILL and freeze, and it
> drops the wait use per Oleg's review comment that it's no longer needed
> when using CLONE_THREAD.
>
> Fixes: 6e890c5d5021 ("vhost: use vhost_tasks for worker threads")
> Signed-off-by: Mike Christie <michael.christie@xxxxxxxxxx>
> ---
> drivers/vhost/vhost.c | 17 ++++++++++++++++-
> include/linux/sched/task.h | 2 +-
> kernel/fork.c | 12 +++---------
> kernel/signal.c | 1 +
> kernel/vhost_task.c | 16 ++++------------
> 5 files changed, 25 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index a92af08e7864..bf83e9340e40 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -338,6 +338,7 @@ static int vhost_worker(void *data)
> struct vhost_worker *worker = data;
> struct vhost_work *work, *work_next;
> struct llist_node *node;
> + bool dead = false;
>
> for (;;) {
> /* mb paired w/ kthread_stop */
> @@ -349,8 +350,22 @@ static int vhost_worker(void *data)
> }
>
> node = llist_del_all(&worker->work_list);
> - if (!node)
> + if (!node) {
> schedule();
> + /*
> + * When we get a SIGKILL our release function will
> + * be called. That will stop new IOs from being queued
> + * and check for outstanding cmd responses. It will then
> + * call vhost_task_stop to tell us to return and exit.
> + */
> + if (!dead && signal_pending(current)) {
> + struct ksignal ksig;
> +
> + dead = get_signal(&ksig);
> + if (dead)
> + clear_thread_flag(TIF_SIGPENDING);


Does get_signal actually return true only on SIGKILL then?

> + }
> + }
>
> node = llist_reverse_order(node);
> /* make sure flag is seen after deletion */
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index 537cbf9a2ade..249a5ece9def 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -29,7 +29,7 @@ struct kernel_clone_args {
> u32 io_thread:1;
> u32 user_worker:1;
> u32 no_files:1;
> - u32 ignore_signals:1;
> + u32 block_signals:1;
> unsigned long stack;
> unsigned long stack_size;
> unsigned long tls;
> diff --git a/kernel/fork.c b/kernel/fork.c
> index ed4e01daccaa..9e04ab5c3946 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2338,14 +2338,10 @@ __latent_entropy struct task_struct *copy_process(
> p->flags |= PF_KTHREAD;
> if (args->user_worker)
> p->flags |= PF_USER_WORKER;
> - if (args->io_thread) {
> - /*
> - * Mark us an IO worker, and block any signal that isn't
> - * fatal or STOP
> - */
> + if (args->io_thread)
> p->flags |= PF_IO_WORKER;
> + if (args->block_signals)
> siginitsetinv(&p->blocked, sigmask(SIGKILL)|sigmask(SIGSTOP));
> - }
>
> if (args->name)
> strscpy_pad(p->comm, args->name, sizeof(p->comm));
> @@ -2517,9 +2513,6 @@ __latent_entropy struct task_struct *copy_process(
> if (retval)
> goto bad_fork_cleanup_io;
>
> - if (args->ignore_signals)
> - ignore_signals(p);
> -
> stackleak_task_init(p);
>
> if (pid != &init_struct_pid) {
> @@ -2861,6 +2854,7 @@ struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node)
> .fn_arg = arg,
> .io_thread = 1,
> .user_worker = 1,
> + .block_signals = 1,
> };
>
> return copy_process(NULL, 0, node, &args);
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 8050fe23c732..a0f00a078cbb 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2891,6 +2891,7 @@ bool get_signal(struct ksignal *ksig)
>
> return ksig->sig > 0;
> }
> +EXPORT_SYMBOL_GPL(get_signal);

If you are exporting this, could you add documentation please?


> /**
> * signal_delivered - called after signal delivery to update blocked signals
> diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
> index b7cbd66f889e..7a2d7d9fe772 100644
> --- a/kernel/vhost_task.c
> +++ b/kernel/vhost_task.c
> @@ -31,22 +31,13 @@ static int vhost_task_fn(void *data)
> */
> void vhost_task_stop(struct vhost_task *vtsk)
> {
> - pid_t pid = vtsk->task->pid;
> -
> set_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags);
> wake_up_process(vtsk->task);
> /*
> * Make sure vhost_task_fn is no longer accessing the vhost_task before
> - * freeing it below. If userspace crashed or exited without closing,
> - * then the vhost_task->task could already be marked dead so
> - * kernel_wait will return early.
> + * freeing it below.
> */
> wait_for_completion(&vtsk->exited);
> - /*
> - * If we are just closing/removing a device and the parent process is
> - * not exiting then reap the task.
> - */
> - kernel_wait4(pid, NULL, __WCLONE, NULL);
> kfree(vtsk);
> }
> EXPORT_SYMBOL_GPL(vhost_task_stop);
> @@ -75,13 +66,14 @@ struct vhost_task *vhost_task_create(int (*fn)(void *), void *arg,
> const char *name)
> {
> struct kernel_clone_args args = {
> - .flags = CLONE_FS | CLONE_UNTRACED | CLONE_VM,
> + .flags = CLONE_FS | CLONE_UNTRACED | CLONE_VM |
> + CLONE_THREAD | CLONE_SIGHAND,
> .exit_signal = 0,
> .fn = vhost_task_fn,
> .name = name,
> .user_worker = 1,
> .no_files = 1,
> - .ignore_signals = 1,
> + .block_signals = 1,
> };
> struct vhost_task *vtsk;
> struct task_struct *tsk;
> --
> 2.25.1