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@xxxxxxxxxx/ > > Cc: luto@xxxxxxxxxxxxxx > Cc: rostedt@xxxxxxxxxxx > Cc: dancol@xxxxxxxxxx > Cc: christian@xxxxxxxxxx > Cc: jannh@xxxxxxxxxx > Cc: surenb@xxxxxxxxxx > Cc: torvalds@xxxxxxxxxxxxxxxxxxxx > Co-developed-by: Daniel Colascione <dancol@xxxxxxxxxx> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > > --- > 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 >
