On Sat, Mar 16, 2019 at 09:53:06PM -0400, Joel Fernandes wrote: > On Sat, Mar 16, 2019 at 12:37:18PM -0700, Suren Baghdasaryan wrote: > > On Sat, Mar 16, 2019 at 11:57 AM Christian Brauner <christian@xxxxxxxxxx> wrote: > > > > > > On Sat, Mar 16, 2019 at 11:00:10AM -0700, Daniel Colascione wrote: > > > > On Sat, Mar 16, 2019 at 10:31 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > > > > > > > > On Fri, Mar 15, 2019 at 11:49 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > On Fri, Mar 15, 2019 at 07:24:28PM +0100, Christian Brauner wrote: > > > > > > [..] > > > > > > > > why do we want to add a new syscall (pidfd_wait) though? Why not just use > > > > > > > > standard poll/epoll interface on the proc fd like Daniel was suggesting. > > > > > > > > AFAIK, once the proc file is opened, the struct pid is essentially pinned > > > > > > > > even though the proc number may be reused. Then the caller can just poll. > > > > > > > > We can add a waitqueue to struct pid, and wake up any waiters on process > > > > > > > > death (A quick look shows task_struct can be mapped to its struct pid) and > > > > > > > > also possibly optimize it using Steve's TIF flag idea. No new syscall is > > > > > > > > needed then, let me know if I missed something? > > > > > > > > > > > > > > Huh, I thought that Daniel was against the poll/epoll solution? > > > > > > > > > > > > Hmm, going through earlier threads, I believe so now. Here was Daniel's > > > > > > reasoning about avoiding a notification about process death through proc > > > > > > directory fd: http://lkml.iu.edu/hypermail/linux/kernel/1811.0/00232.html > > > > > > > > > > > > May be a dedicated syscall for this would be cleaner after all. > > > > > > > > > > Ah, I wish I've seen that discussion before... > > > > > syscall makes sense and it can be non-blocking and we can use > > > > > select/poll/epoll if we use eventfd. > > > > > > > > Thanks for taking a look. > > > > > > > > > I would strongly advocate for > > > > > non-blocking version or at least to have a non-blocking option. > > > > > > > > Waiting for FD readiness is *already* blocking or non-blocking > > > > according to the caller's desire --- users can pass options they want > > > > to poll(2) or whatever. There's no need for any kind of special > > > > configuration knob or non-blocking option. We already *have* a > > > > non-blocking option that works universally for everything. > > > > > > > > As I mentioned in the linked thread, waiting for process exit should > > > > work just like waiting for bytes to appear on a pipe. Process exit > > > > status is just another blob of bytes that a process might receive. A > > > > process exit handle ought to be just another information source. The > > > > reason the unix process API is so awful is that for whatever reason > > > > the original designers treated processes as some kind of special kind > > > > of resource instead of fitting them into the otherwise general-purpose > > > > unix data-handling API. Let's not repeat that mistake. > > > > > > > > > Something like this: > > > > > > > > > > evfd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC); > > > > > // register eventfd to receive death notification > > > > > pidfd_wait(pid_to_kill, evfd); > > > > > // kill the process > > > > > pidfd_send_signal(pid_to_kill, ...) > > > > > // tend to other things > > > > > > > > Now you've lost me. pidfd_wait should return a *new* FD, not wire up > > > > an eventfd. > > > > > > > > Ok, I probably misunderstood your post linked by Joel. I though your > > original proposal was based on being able to poll a file under > > /proc/pid and then you changed your mind to have a separate syscall > > which I assumed would be a blocking one to wait for process exit. > > Maybe you can describe the new interface you are thinking about in > > terms of userspace usage like I did above? Several lines of code would > > explain more than paragraphs of text. > > Hey, Thanks Suren for the eventfd idea. I agree with Daniel on this. The idea > from Daniel here is to wait for process death and exit events by just > referring to a stable fd, independent of whatever is going on in /proc. > > What is needed is something like this (in highly pseudo-code form): > > pidfd = opendir("/proc/<pid>",..); > wait_fd = pidfd_wait(pidfd); > read or poll wait_fd (non-blocking or blocking whichever) > > wait_fd will block until the task has either died or reaped. In both these > cases, it can return a suitable string such as "dead" or "reaped" although an > integer with some predefined meaning is also Ok. > > What that guarantees is, even if the task's PID has been reused, or the task > has already died or already died + reaped, all of these events cannot race > with the code above and the information passed to the user is race-free and > stable / guaranteed. > > An eventfd seems to not fit well, because AFAICS passing the raw PID to > eventfd as in your example would still race since the PID could have been > reused by another process by the time the eventfd is created. > > Also Andy's idea in [1] seems to use poll flags to communicate various tihngs > which is still not as explicit about the PID's status so that's a poor API > choice compared to the explicit syscall. > > I am planning to work on a prototype patch based on Daniel's idea and post something > soon (chatted with Daniel about it and will reference him in the posting as > well), during this posting I will also summarize all the previous discussions > and come up with some tests as well. I hope to have something soon. Having pidfd_wait() return another fd will make the syscall harder to swallow for a lot of people I reckon. What exactly prevents us from making the pidfd itself readable/pollable for the exit staus? They are "special" fds anyway. I would really like to avoid polluting the api with multiple different types of fds if possible. ret = pidfd_wait(pidfd); read or poll pidfd (Note that I'm traveling so my responses might be delayed quite a bit.) (Ccing a few people that might have an opinion here.) Christian
