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. > > Why? Because the new type FD can report process exit *status* > > information (via read(2) after readability signal) as well as this > > binary yes-or-no signal *that* a process exited, and this capability > > is useful if you want to the pidfd interface to be a good > > general-purpose process management facility to replace the awful > > wait() family of functions. You can't get an exit status from an > > eventfd. Wiring up an eventfd the way you've proposed also complicates > > wait-causality information, complicating both tracing and any priority > > inheritance we might want in the future (because all the wakeups gets > > mixed into the eventfd and you can't unscramble an egg). And for what? > > What do we gain by using an eventfd? Is the reason that exit.c would > > be able to use eventfd_signal instead of poking a waitqueue directly? > > How is that better? With an eventfd, you've increased path length on > > process exit *and* complicated the API for no reason. > > > > > ... > > > // wait for the process to die > > > poll_wait(evfd, ...); > > > > > > This simplifies userspace > > > > Not relative to an exit handle it doesn't. > > > > >, allows it to wait for multiple events using > > > epoll > > > > So does a process exit status handle. > > > > > and I think kernel implementation will be also quite simple > > > because it already implements eventfd_signal() that takes care of > > > waitqueue handling. > > > > What if there are multiple eventfds registered for the death of a > > process? In any case, you need some mechanism to find, upon process > > death, a list of waiters, then wake each of them up. That's either a > > global search or a search in some list rooted in a task-related > > structure (either struct task or one of its friends). Using an eventfd > > here adds nothing, since upon death, you need this list search > > regardless, and as I mentioned above, eventfd-wiring just makes the > > API worse. > > > > > If pidfd_send_signal could be extended to have an optional eventfd > > > parameter then we would not even have to add a new syscall. > > > > There is nothing wrong with adding a new system call. I don't know why > > there's this idea circulating that adding system calls is something we > > should bend over backwards to avoid. It's cheap, and support-wise, > > kernel interface is kernel interface. Sending a signal has *nothing* > > to do with wiring up some kind of notification and there's no reason > > to mingle it with some kind of event registration. > > > I agree with Daniel. > One design goal is to not stuff clearly delinated tasks related to > process management into the same syscall. That will just leave us with a > confusing api. Sending signals is part of managing a process while it is > running. Waiting on a process to end is clearly separate from that. > It's important to keep in mind that the goal of the pidfd work is to end > up with an api that is of use to all of user space concerned with > process management not just a specific project. I'm not bent on adding or not adding a new syscall as long as functionality is there. Thanks! _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
