From: joel at joelfernandes.org (Joel Fernandes) Subject: [PATCH v1 1/2] Add polling support to pidfd Date: Mon, 29 Apr 2019 10:02:45 -0400 [thread overview] Message-ID: <20190429140245.GB233442@google.com> (raw) In-Reply-To: <20190428162405.GA6757@redhat.com> On Sun, Apr 28, 2019 at 06:24:06PM +0200, Oleg Nesterov wrote: > Thanks for cc'ing me... > > On 04/26, Christian Brauner wrote: > > > > On Thu, Apr 25, 2019 at 03:00:09PM -0400, Joel Fernandes (Google) wrote: > > > +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; > > Joel, I still can't understand why do we need tasklist... and I don't really > understand the comment. The code looks as if you are trying to avoid poll_wait(), > but this would be strange. > > OK, why can't pidfd_poll() do > > poll_wait(file, &pid->wait_pidfd, pts); > > rcu_read_lock(); > task = pid_task(pid, PIDTYPE_PID); > if (!task || task->exit_state && thread_group_empty(task)) > poll_flags = POLLIN | ...; > rcu_read_unlock(); > > return poll_flags; > > ? Oh that's much better Oleg, and would avoid the race I had in mind: Basically I was acquiring the tasklist_lock to avoid a case where a polling task is not woken up because it was added to the waitqueue too late. The reading of the exit_state and the conditional adding to the wait queue, needed to be atomic. Otherwise something like the following may be possible: Task A (poller) Task B (exiting task being polled) ------------ ---------------- poll() called exit_state is set to non-zero read exit_state wake_up_all() add_wait_queue() ---------------------------------------------- However, in your code above, it is avoided because we get: Task A (poller) Task B (exiting task being polled) ------------ ---------------- poll() called add_wait_queue() exit_state is set to non-zero read exit_state remove_wait_queue() wake_up_all() I don't see any other issues with your code above so I can try it out and update the patches. Thanks. > > > +static void do_notify_pidfd(struct task_struct *task) > > > > Maybe a short command that this helper can only be called when we know > > that task is a thread-group leader wouldn't hurt so there's no confusion > > later. > > Not really. If the task is traced, do_notify_parent() (and thus do_notify_pidfd()) > can be called to notify the debugger even if the task is not a leader and/or if > it is not the last thread. The latter means a spurious wakeup for pidfd_poll(). Seems like you are replying to Christian's point. I agree with you. > > > +{ > > > + struct pid *pid; > > > + > > > + lockdep_assert_held(&tasklist_lock); > > > + > > > + pid = get_task_pid(task, PIDTYPE_PID); > > > + wake_up_all(&pid->wait_pidfd); > > > + put_pid(pid); > > Why get/put? Yes, pid_task() should do it. Will update it. Thanks! - Joel
WARNING: multiple messages have this Message-ID (diff)
From: joel@joelfernandes.org (Joel Fernandes) Subject: [PATCH v1 1/2] Add polling support to pidfd Date: Mon, 29 Apr 2019 10:02:45 -0400 [thread overview] Message-ID: <20190429140245.GB233442@google.com> (raw) Message-ID: <20190429140245.jdA0CEhoxH3n6GFgSPJtHiGtP1c0waBbqCfzJC-mFcY@z> (raw) In-Reply-To: <20190428162405.GA6757@redhat.com> On Sun, Apr 28, 2019@06:24:06PM +0200, Oleg Nesterov wrote: > Thanks for cc'ing me... > > On 04/26, Christian Brauner wrote: > > > > On Thu, Apr 25, 2019@03:00:09PM -0400, Joel Fernandes (Google) wrote: > > > +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; > > Joel, I still can't understand why do we need tasklist... and I don't really > understand the comment. The code looks as if you are trying to avoid poll_wait(), > but this would be strange. > > OK, why can't pidfd_poll() do > > poll_wait(file, &pid->wait_pidfd, pts); > > rcu_read_lock(); > task = pid_task(pid, PIDTYPE_PID); > if (!task || task->exit_state && thread_group_empty(task)) > poll_flags = POLLIN | ...; > rcu_read_unlock(); > > return poll_flags; > > ? Oh that's much better Oleg, and would avoid the race I had in mind: Basically I was acquiring the tasklist_lock to avoid a case where a polling task is not woken up because it was added to the waitqueue too late. The reading of the exit_state and the conditional adding to the wait queue, needed to be atomic. Otherwise something like the following may be possible: Task A (poller) Task B (exiting task being polled) ------------ ---------------- poll() called exit_state is set to non-zero read exit_state wake_up_all() add_wait_queue() ---------------------------------------------- However, in your code above, it is avoided because we get: Task A (poller) Task B (exiting task being polled) ------------ ---------------- poll() called add_wait_queue() exit_state is set to non-zero read exit_state remove_wait_queue() wake_up_all() I don't see any other issues with your code above so I can try it out and update the patches. Thanks. > > > +static void do_notify_pidfd(struct task_struct *task) > > > > Maybe a short command that this helper can only be called when we know > > that task is a thread-group leader wouldn't hurt so there's no confusion > > later. > > Not really. If the task is traced, do_notify_parent() (and thus do_notify_pidfd()) > can be called to notify the debugger even if the task is not a leader and/or if > it is not the last thread. The latter means a spurious wakeup for pidfd_poll(). Seems like you are replying to Christian's point. I agree with you. > > > +{ > > > + struct pid *pid; > > > + > > > + lockdep_assert_held(&tasklist_lock); > > > + > > > + pid = get_task_pid(task, PIDTYPE_PID); > > > + wake_up_all(&pid->wait_pidfd); > > > + put_pid(pid); > > Why get/put? Yes, pid_task() should do it. Will update it. Thanks! - Joel
next prev parent reply other threads:[~2019-04-29 14:02 UTC|newest] Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-04-25 19:00 [PATCH v1 1/2] Add polling support to pidfd joel 2019-04-25 19:00 ` Joel Fernandes (Google) 2019-04-25 19:00 ` [PATCH v1 2/2] Add selftests for pidfd polling joel 2019-04-25 19:00 ` Joel Fernandes (Google) 2019-04-25 20:00 ` tycho 2019-04-25 20:00 ` Tycho Andersen 2019-04-26 13:47 ` joel 2019-04-26 13:47 ` Joel Fernandes 2019-04-25 21:29 ` christian 2019-04-25 21:29 ` Christian Brauner 2019-04-25 22:07 ` dancol 2019-04-25 22:07 ` Daniel Colascione 2019-04-26 17:26 ` joel 2019-04-26 17:26 ` Joel Fernandes 2019-04-26 19:35 ` dancol 2019-04-26 19:35 ` Daniel Colascione 2019-04-26 20:31 ` joel 2019-04-26 20:31 ` Joel Fernandes 2019-04-26 13:42 ` joel 2019-04-26 13:42 ` Joel Fernandes 2019-04-25 22:24 ` [PATCH v1 1/2] Add polling support to pidfd christian 2019-04-25 22:24 ` Christian Brauner 2019-04-26 14:23 ` joel 2019-04-26 14:23 ` Joel Fernandes 2019-04-26 15:21 ` christian 2019-04-26 15:21 ` Christian Brauner 2019-04-26 15:31 ` christian 2019-04-26 15:31 ` Christian Brauner 2019-04-28 16:24 ` oleg 2019-04-28 16:24 ` Oleg Nesterov 2019-04-29 14:02 ` joel [this message] 2019-04-29 14:02 ` Joel Fernandes 2019-04-29 14:07 ` joel 2019-04-29 14:07 ` Joel Fernandes 2019-04-29 14:25 ` oleg 2019-04-29 14:25 ` Oleg Nesterov 2019-04-29 14:20 ` oleg 2019-04-29 14:20 ` Oleg Nesterov 2019-04-29 16:32 ` joel 2019-04-29 16:32 ` Joel Fernandes 2019-04-30 11:53 ` oleg 2019-04-30 11:53 ` Oleg Nesterov 2019-04-30 12:07 ` oleg 2019-04-30 12:07 ` Oleg Nesterov 2019-04-30 15:49 ` joel 2019-04-30 15:49 ` Joel Fernandes 2019-04-26 14:58 ` christian 2019-04-26 14:58 ` Christian Brauner
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