On Sun, Mar 17, 2019 at 9:35 AM Serge E. Hallyn <serge@xxxxxxxxxx> wrote: > > On Sun, Mar 17, 2019 at 12:42:40PM +0100, Christian Brauner wrote: > > 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 > > I'm not quite clear on what the two steps are doing here. Is pidfd_wait() > doing a waitpid(2), and the read gets exit status info? pidfd_wait on an open pidfd returns a "wait handle" FD. The wait handle works just like a pipe: you can select/epoll/whatever for readability. read(2) on the wait handle (which blocks unless you set O_NONBLOCK, just like a pipe) completes with a siginfo_t when the process to which the wait handle is attached exits. Roughly, int kill_and_wait_for_exit(int pidfd) { int wait_handle = pidfd_wait(pidfd); pidfd_send_signal(pidfd, ...); siginfo_t exit_info; read(wait_handle, &exit_info, sizeof(exit_info)); // Blocks because we haven't configured non-blocking behavior, just like a pipe. close(wait_handle); return exit_info.si_status; } > > > (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 > > On its own, what you (Christian) show seems nicer. But think about a main event > loop (like in lxc), where we just loop over epoll_wait() on various descriptors. > If we want to wait for any of several types of events - maybe a signalfd, socket > traffic, or a process death - it would be nice if we can treat them all the same > way, without having to setup a separate thread to watch the pidfd and send > data over another fd. Is there a nice way we can provide that with what you've > got above? Nobody is proposing any kind of mechanism that would require a separate thread. What I'm proposing works with poll and read and should be trivial to integrate into any existing event loop: from the perspective of the event loop, it looks just like a pipe. _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
