lkml.org 
[lkml]   [2019]   [Apr]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v1 1/2] Add polling support to pidfd
On Thu, Apr 25, 2019 at 03:00:09PM -0400, Joel Fernandes (Google) wrote:
> pidfd are file descriptors referring to a process created with the
> CLONE_PIDFD clone(2) flag. Android low memory killer (LMK) needs pidfd
> polling support to replace code that currently checks for existence of
> /proc/pid for knowing that a process that is signalled to be killed has
> died, which is both racy and slow. The pidfd poll approach is race-free,
> and also allows the LMK to do other things (such as by polling on other
> fds) while awaiting the process being killed to die.
>
> It prevents a situation where a PID is reused between when LMK sends a
> kill signal and checks for existence of the PID, since the wrong PID is
> now possibly checked for existence.
>
> In this patch, we follow the same existing mechanism in the kernel used
> when the parent of the task group is to be notified (do_notify_parent).
> This is when the tasks waiting on a poll of pidfd are also awakened.
>
> We have decided to include the waitqueue in struct pid for the following
> reasons:
> 1. The wait queue has to survive for the lifetime of the poll. Including
> it in task_struct would not be option in this case because the task can
> be reaped and destroyed before the poll returns.
>
> 2. By including the struct pid for the waitqueue means that during
> de_thread(), the new thread group leader automatically gets the new
> waitqueue/pid even though its task_struct is different.
>
> Appropriate test cases are added in the second patch to provide coverage
> of all the cases the patch is handling.
>
> Andy had a similar patch [1] in the past which was a good reference
> however this patch tries to handle different situations properly related
> to thread group existence, and how/where it notifies. And also solves
> other bugs (waitqueue lifetime). Daniel had a similar patch [2]
> recently which this patch supercedes.
>
> [1] https://lore.kernel.org/patchwork/patch/345098/
> [2] https://lore.kernel.org/lkml/20181029175322.189042-1-dancol@google.com/
>
> Cc: luto@amacapital.net
> Cc: rostedt@goodmis.org
> Cc: dancol@google.com
> Cc: sspatil@google.com
> Cc: christian@brauner.io
> Cc: jannh@google.com
> Cc: surenb@google.com
> Cc: timmurray@google.com
> Cc: Jonathan Kowalski <bl0pbl33p@gmail.com>
> Cc: torvalds@linux-foundation.org
> Cc: kernel-team@android.com

That should be of the form:

Cc: First Name <email@address.com>


> Co-developed-by: Daniel Colascione <dancol@google.com>

Every CDB needs to come with a SOB.

> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>
> ---
>
> RFC -> v1:
> * Based on CLONE_PIDFD patches: https://lwn.net/Articles/786244/
> * Updated selftests.
> * Renamed poll wake function to do_notify_pidfd.
> * Removed depending on EXIT flags
> * Removed POLLERR flag since semantics are controversial and
> we don't have usecases for it right now (later we can add if there's
> a need for it).
>
> include/linux/pid.h | 3 +++
> kernel/fork.c | 33 +++++++++++++++++++++++++++++++++
> kernel/pid.c | 2 ++
> kernel/signal.c | 14 ++++++++++++++
> 4 files changed, 52 insertions(+)
>
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index 3c8ef5a199ca..1484db6ca8d1 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -3,6 +3,7 @@
> #define _LINUX_PID_H
>
> #include <linux/rculist.h>
> +#include <linux/wait.h>
>
> enum pid_type
> {
> @@ -60,6 +61,8 @@ struct pid
> unsigned int level;
> /* lists of tasks that use this pid */
> struct hlist_head tasks[PIDTYPE_MAX];
> + /* wait queue for pidfd notifications */
> + wait_queue_head_t wait_pidfd;
> struct rcu_head rcu;
> struct upid numbers[1];
> };
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 5525837ed80e..fb3b614f6456 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1685,8 +1685,41 @@ static void pidfd_show_fdinfo(struct seq_file *m, struct file *f)
> }
> #endif
>
> +static unsigned int pidfd_poll(struct file *file, struct poll_table_struct *pts)
> +{
> + struct task_struct *task;
> + struct pid *pid;
> + int poll_flags = 0;
> +
> + /*
> + * tasklist_lock must be held because to avoid racing with
> + * changes in exit_state and wake up. Basically to avoid:
> + *
> + * P0: read exit_state = 0
> + * P1: write exit_state = EXIT_DEAD
> + * P1: Do a wake up - wq is empty, so do nothing
> + * P0: Queue for polling - wait forever.
> + */
> + read_lock(&tasklist_lock);
> + pid = file->private_data;
> + task = pid_task(pid, PIDTYPE_PID);
> + WARN_ON_ONCE(task && !thread_group_leader(task));
> +
> + if (!task || (task->exit_state && thread_group_empty(task)))
> + poll_flags = POLLIN | POLLRDNORM;
> +
> + if (!poll_flags)
> + poll_wait(file, &pid->wait_pidfd, pts);
> +
> + read_unlock(&tasklist_lock);
> +
> + return poll_flags;
> +}
> +
> +
> const struct file_operations pidfd_fops = {
> .release = pidfd_release,
> + .poll = pidfd_poll,
> #ifdef CONFIG_PROC_FS
> .show_fdinfo = pidfd_show_fdinfo,
> #endif
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 20881598bdfa..5c90c239242f 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -214,6 +214,8 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> for (type = 0; type < PIDTYPE_MAX; ++type)
> INIT_HLIST_HEAD(&pid->tasks[type]);
>
> + init_waitqueue_head(&pid->wait_pidfd);
> +
> upid = pid->numbers + ns->level;
> spin_lock_irq(&pidmap_lock);
> if (!(ns->pid_allocated & PIDNS_ADDING))
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 1581140f2d99..16e7718316e5 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1800,6 +1800,17 @@ int send_sigqueue(struct sigqueue *q, struct pid *pid, enum pid_type type)
> return ret;
> }
>
> +static void do_notify_pidfd(struct task_struct *task)
> +{
> + struct pid *pid;
> +
> + lockdep_assert_held(&tasklist_lock);
> +
> + pid = get_task_pid(task, PIDTYPE_PID);
> + wake_up_all(&pid->wait_pidfd);
> + put_pid(pid);
> +}
> +
> /*
> * Let a parent know about the death of a child.
> * For a stopped/continued status change, use do_notify_parent_cldstop instead.
> @@ -1823,6 +1834,9 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
> BUG_ON(!tsk->ptrace &&
> (tsk->group_leader != tsk || !thread_group_empty(tsk)));
>
> + /* Wake up all pidfd waiters */
> + do_notify_pidfd(tsk);
> +
> if (sig != SIGCHLD) {
> /*
> * This is only possible if parent == real_parent.
> --
> 2.21.0.593.g511ec345e18-goog

\
 
 \ /
  Last update: 2019-04-26 16:59    [W:0.188 / U:31.452 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site