On Sat, Apr 20, 2019 at 12:08 AM Daniel Colascione <dan...@google.com> wrote: > > On Fri, Apr 19, 2019 at 2:45 PM Joel Fernandes <j...@joelfernandes.org> wrote: > > > > On Fri, Apr 19, 2019 at 02:24:09PM -0700, Daniel Colascione wrote: > > > On Fri, Apr 19, 2019 at 2:20 PM Joel Fernandes <j...@joelfernandes.org> > > > wrote: > > > > > > > > On Fri, Apr 19, 2019 at 10:57:11PM +0200, Christian Brauner wrote: > > > > > On Fri, Apr 19, 2019 at 10:34 PM Daniel Colascione > > > > > <dan...@google.com> wrote: > > > > > > > > > > > > On Fri, Apr 19, 2019 at 12:49 PM Joel Fernandes > > > > > > <j...@joelfernandes.org> wrote: > > > > > > > > > > > > > > On Fri, Apr 19, 2019 at 09:18:59PM +0200, Christian Brauner wrote: > > > > > > > > On Fri, Apr 19, 2019 at 03:02:47PM -0400, Joel Fernandes wrote: > > > > > > > > > On Thu, Apr 18, 2019 at 07:26:44PM +0200, Christian Brauner > > > > > > > > > wrote: > > > > > > > > > > On April 18, 2019 7:23:38 PM GMT+02:00, Jann Horn > > > > > > > > > > <ja...@google.com> wrote: > > > > > > > > > > >On Wed, Apr 17, 2019 at 3:09 PM Oleg Nesterov > > > > > > > > > > ><o...@redhat.com> wrote: > > > > > > > > > > >> On 04/16, Joel Fernandes wrote: > > > > > > > > > > >> > On Tue, Apr 16, 2019 at 02:04:31PM +0200, Oleg > > > > > > > > > > >> > Nesterov wrote: > > > > > > > > > > >> > > > > > > > > > > > > >> > > Could you explain when it should return POLLIN? When > > > > > > > > > > >> > > the whole > > > > > > > > > > >process exits? > > > > > > > > > > >> > > > > > > > > > > > >> > It returns POLLIN when the task is dead or doesn't > > > > > > > > > > >> > exist anymore, > > > > > > > > > > >or when it > > > > > > > > > > >> > is in a zombie state and there's no other thread in > > > > > > > > > > >> > the thread > > > > > > > > > > >group. > > > > > > > > > > >> > > > > > > > > > > >> IOW, when the whole thread group exits, so it can't be > > > > > > > > > > >> used to > > > > > > > > > > >monitor sub-threads. > > > > > > > > > > >> > > > > > > > > > > >> just in case... speaking of this patch it doesn't modify > > > > > > > > > > >proc_tid_base_operations, > > > > > > > > > > >> so you can't poll("/proc/sub-thread-tid") anyway, but > > > > > > > > > > >> iiuc you are > > > > > > > > > > >going to use > > > > > > > > > > >> the anonymous file returned by CLONE_PIDFD ? > > > > > > > > > > > > > > > > > > > > > >I don't think procfs works that way. /proc/sub-thread-tid > > > > > > > > > > >has > > > > > > > > > > >proc_tgid_base_operations despite not being a thread group > > > > > > > > > > >leader. > > > > > > > > > > >(Yes, that's kinda weird.) AFAICS the WARN_ON_ONCE() in > > > > > > > > > > >this code can > > > > > > > > > > >be hit trivially, and then the code will misbehave. > > > > > > > > > > > > > > > > > > > > > >@Joel: I think you'll have to either rewrite this to > > > > > > > > > > >explicitly bail > > > > > > > > > > >out if you're dealing with a thread group leader, or make > > > > > > > > > > >the code > > > > > > > > > > >work for threads, too. > > > > > > > > > > > > > > > > > > > > The latter case probably being preferred if this API is > > > > > > > > > > supposed to be > > > > > > > > > > useable for thread management in userspace. > > > > > > > > > > > > > > > > > > At the moment, we are not planning to use this for sub-thread > > > > > > > > > management. I > > > > > > > > > am reworking this patch to only work on clone(2) pidfds which > > > > > > > > > makes the above > > > > > > > > > > > > > > > > Indeed and agreed. > > > > > > > > > > > > > > > > > discussion about /proc a bit unnecessary I think. Per the > > > > > > > > > latest CLONE_PIDFD > > > > > > > > > patches, CLONE_THREAD with pidfd is not supported. > > > > > > > > > > > > > > > > Yes. We have no one asking for it right now and we can easily > > > > > > > > add this > > > > > > > > later. > > > > > > > > > > > > > > > > Admittedly I haven't gotten around to reviewing the patches > > > > > > > > here yet > > > > > > > > completely. But one thing about using POLLIN. FreeBSD is using > > > > > > > > POLLHUP > > > > > > > > on process exit which I think is nice as well. How about > > > > > > > > returning > > > > > > > > POLLIN | POLLHUP on process exit? > > > > > > > > We already do things like this. For example, when you proxy > > > > > > > > between > > > > > > > > ttys. If the process that you're reading data from has exited > > > > > > > > and closed > > > > > > > > it's end you still can't usually simply exit because it might > > > > > > > > have still > > > > > > > > buffered data that you want to read. The way one can deal with > > > > > > > > this > > > > > > > > from userspace is that you can observe a (POLLHUP | POLLIN) > > > > > > > > event and > > > > > > > > you keep on reading until you only observe a POLLHUP without a > > > > > > > > POLLIN > > > > > > > > event at which point you know you have read > > > > > > > > all data. > > > > > > > > I like the semantics for pidfds as well as it would indicate: > > > > > > > > - POLLHUP -> process has exited > > > > > > > > - POLLIN -> information can be read > > > > > > > > > > > > > > Actually I think a bit different about this, in my opinion the > > > > > > > pidfd should > > > > > > > always be readable (we would store the exit status somewhere in > > > > > > > the future > > > > > > > which would be readable, even after task_struct is dead). So I > > > > > > > was thinking > > > > > > > we always return EPOLLIN. If process has not exited, then it > > > > > > > blocks. > > > > > > > > > > > > ITYM that a pidfd polls as readable *once a task exits* and stays > > > > > > readable forever. Before a task exit, a poll on a pidfd should *not* > > > > > > yield POLLIN and reading that pidfd should *not* complete > > > > > > immediately. > > > > > > There's no way that, having observed POLLIN on a pidfd, you should > > > > > > ever then *not* see POLLIN on that pidfd in the future --- it's a > > > > > > one-way transition from not-ready-to-get-exit-status to > > > > > > ready-to-get-exit-status. > > > > > > > > > > What do you consider interesting state transitions? A listener on a > > > > > pidfd > > > > > in epoll_wait() might be interested if the process execs for example. > > > > > That's a very valid use-case for e.g. systemd. > > > > > We can't use EPOLLIN for that too otherwise you'd need to to > > > > > waitid(_WNOHANG) > > > > > to check whether an exit status can be read which is not nice and > > > > > then you > > > > > multiplex different meanings on the same bit. > > > > > I would prefer if the exit status can only be read from the parent > > > > > which is > > > > > clean and the least complicated semantics, i.e. Linus waitid() idea. > > > > > EPOLLIN on a pidfd could very well mean that data can be read via > > > > > a read() on the pidfd *other* than the exit status. The read could > > > > > e.g. > > > > > give you a lean struct that indicates the type of state transition: > > > > > NOTIFY_EXIT, > > > > > NOTIFY_EXEC, etc.. This way we are not bound to a specific poll event > > > > > indicating > > > > > a specific state. > > > > > Though there's a case to be made that EPOLLHUP could indicate process > > > > > exit > > > > > and EPOLLIN a state change + read(). > > > > > > > > According to Linus, POLLHUP usually indicates that something is > > > > readable: > > > > > > I don't think Linus said that POLLHUP means readable. He did say that > > > it usually doesn't make sense to set POLLHUP without POLLIN, but > > > that's not the same as POLLHUP indicating readability. > > > > Ok, fair enough. > > > > > > https://lkml.org/lkml/2019/4/18/1181 > > > > "So generally a HUP condition should mean that POLLIN and POLLOUT also > > > > get set. Not because there's any actual _data_ to be read, but simply > > > > because the read will not block." > > > > > > > > I feel the future state changes such as for NOTIFY_EXEC can easily be > > > > implemented on top of this patch. > > > > > > > > Just for the exit notification purposes, the states are: > > > > if process has exit_state == 0, block. > > > > if process is zombie/dead but not reaped, then return POLLIN > > > > if process is reaped, then return POLLIN | POLLHUP > > > > > > Setting POLLHUP when the process is reaped is harmless, but I don't > > > think it's useful. I can't think of a reason that anyone would care. > > > > We can also outright remove it. Oleg seemed to not mind it, in fact he said > > it may be useful to indicate the reap status so at least I am inclined to > > leave it in. > > > > > You can't block and wait on reaping, so you could only busy-wait, and > > > you can look for ESRCH on any proc file today to detect reaping. I'd > > > > proc file reading is racy though. We shouldn't even talk about that since > > the > > point of pidfd is to avoid such raw "pid" related races. > > It's not racy if you have a procfs dirfd handle open anyway. But since > we moved to the two-kinds-of-file-descriptor model, there's no way to > reliably open the procfs directory FD for a process that's exited but > that's not yet been reaped. The fdinfo mechanism doesn't solve this
Uhm, maybe I'm slow but why? static int do_child(void *p) { exit(EXIT_SUCCESS); } pid_t pid = clone(do_child, CLONE_PIDFD, &pidfd); sleep(10); /* process exits but is not reaped yet */ procfd = open("/proc/pid"); if (pidfd_send_signal(pidfd, 0, NULL, 0) == 0) /* procfd is valid */ waitpid(pid); > problem. But adding a reap flag to poll output doesn't close the race > either. > > I don't see any actual benefit to knowing via poll whether a process > was reaped. If you can identify such a use case, great. Otherwise, I > want to keep the poll interface simple and not specify semantics for > POLLHUP.