On Fri, Apr 19, 2019 at 11:48 PM Christian Brauner christian@brauner.io wrote:
On Fri, Apr 19, 2019 at 11:21 PM Daniel Colascione dancol@google.com wrote:
On Fri, Apr 19, 2019 at 1:57 PM Christian Brauner christian@brauner.io wrote:
On Fri, Apr 19, 2019 at 10:34 PM Daniel Colascione dancol@google.com wrote:
On Fri, Apr 19, 2019 at 12:49 PM Joel Fernandes joel@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 jannh@google.com wrote: > > >On Wed, Apr 17, 2019 at 3:09 PM Oleg Nesterov oleg@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.
Sure, but systemd is specialized.
So is Android and we're not designing an interface for Android but for all of userspace. I hope this is clear. Service managers are quite important and systemd is the largest one and they can make good use of this feature.
There are two broad classes of programs that care about process exit status: 1) those that just want to do something and wait for it to complete, and 2) programs that want to perform detailed monitoring of processes and intervention in their state. #1 is overwhelmingly more common. The basic pidfd feature should take care of case #1 only, as wait*() in file descriptor form. I definitely don't think we should be complicating the interface and making it more error-prone (see below) for the sake of that rare program that cares about non-exit notification conditions. You're proposing a complicated combination of poll bit flags that most users (the ones who just wait to wait for processes) don't care about and that risk making the facility hard to use with existing event loops, which generally recognize readability and writability as the only properties that are worth monitoring.
That whole pargraph is about dismissing a range of valid use-cases based on assumptions such as "way more common" and even argues that service managers are special cases and therefore not really worth considering. I would like to be more open to other use cases.
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.
Exit status information should be *at least* as broadly available through pidfds as it is through the last field of /proc/pid/stat today, and probably more broadly. I've been saying for six months now that we need to talk about *who* should have access to exit status information. We haven't had that conversation yet. My preference is to just make exit status information globally available, as FreeBSD seems
Totally aside from whether or not this is a good idea but since you keep bringing this up and I'm really curious about this where is this documented and how does this work, please?