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SubjectRe: [PATCH RFC 1/2] Add polling support to pidfd
On Thu, Apr 11, 2019 at 01:50:42PM -0400, Joel Fernandes (Google) wrote:
> pidfd are /proc/pid directory file descriptors referring to a task group
> leader. Android low memory killer (LMK) needs pidfd polling support to
> replace code that currently checks for existence of /proc/pid for
> knowing 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 appears to me that the "pidfd" now will be an anon inode fd, and not based
on /proc/, based on discussions with Linus. So I'll rework the patches
accordingly. However that is relatively independent of this patch so this
version can also be reviewed before I send out the reworked version.

thanks,

- Joel

>
> 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 mechanism used uhen the parent of the
> task group is to be notified, that 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_exec, the thread doing de_thread() 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 (existence of taks_struct). 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: christian@brauner.io
> Cc: jannh@google.com
> Cc: surenb@google.com
> Cc: torvalds@linux-foundation.org
> Co-developed-by: Daniel Colascione <dancol@google.com>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>
> ---
> fs/proc/base.c | 39 +++++++++++++++++++++++++++++++++++++++
> include/linux/pid.h | 3 +++
> kernel/exit.c | 1 -
> kernel/pid.c | 2 ++
> kernel/signal.c | 14 ++++++++++++++
> 5 files changed, 58 insertions(+), 1 deletion(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 6a803a0b75df..879900082647 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -3069,8 +3069,47 @@ static int proc_tgid_base_readdir(struct file *file, struct dir_context *ctx)
> tgid_base_stuff, ARRAY_SIZE(tgid_base_stuff));
> }
>
> +static unsigned int proc_tgid_base_poll(struct file *file, struct poll_table_struct *pts)
> +{
> + int poll_flags = 0;
> + struct task_struct *task;
> + struct pid *pid;
> +
> + task = get_proc_task(file->f_path.dentry->d_inode);
> +
> + WARN_ON_ONCE(task && !thread_group_leader(task));
> +
> + /*
> + * 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);
> + if (!task)
> + poll_flags = POLLIN | POLLRDNORM | POLLERR;
> + else if (task->exit_state == EXIT_DEAD)
> + poll_flags = POLLIN | POLLRDNORM;
> + else if (task->exit_state == EXIT_ZOMBIE && thread_group_empty(task))
> + poll_flags = POLLIN | POLLRDNORM;
> +
> + if (!poll_flags) {
> + pid = proc_pid(file->f_path.dentry->d_inode);
> + poll_wait(file, &pid->wait_pidfd, pts);
> + }
> + read_unlock(&tasklist_lock);
> +
> + if (task)
> + put_task_struct(task);
> + return poll_flags;
> +}
> +
> static const struct file_operations proc_tgid_base_operations = {
> .read = generic_read_dir,
> + .poll = proc_tgid_base_poll,
> .iterate_shared = proc_tgid_base_readdir,
> .llseek = generic_file_llseek,
> };
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index b6f4ba16065a..2e0dcbc6d14e 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 pollers */
> + wait_queue_head_t wait_pidfd;
> struct rcu_head rcu;
> struct upid numbers[1];
> };
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 2166c2d92ddc..c386ec52687d 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -181,7 +181,6 @@ static void delayed_put_task_struct(struct rcu_head *rhp)
> put_task_struct(tsk);
> }
>
> -
> void release_task(struct task_struct *p)
> {
> struct task_struct *leader;
> 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 f98448cf2def..e3781703ef7e 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_wakeup_pidfd_pollers(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_wakeup_pidfd_pollers(tsk);
> +
> if (sig != SIGCHLD) {
> /*
> * This is only possible if parent == real_parent.
> --
> 2.21.0.392.gf8f6787159e-goog
>

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