On Fri, Mar 13, 2015 at 2:45 PM, <josh@xxxxxxxxxxxxxxxx> wrote: > On Fri, Mar 13, 2015 at 02:33:44PM -0700, Andy Lutomirski wrote: >> On Fri, Mar 13, 2015 at 12:42 PM, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote: >> > On Fri, Mar 13, 2015 at 04:05:29PM +0000, David Drysdale wrote: >> >> On Fri, Mar 13, 2015 at 1:40 AM, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote: >> >> > This patch series introduces a new clone flag, CLONE_FD, which lets the caller >> >> > handle child process exit notification via a file descriptor rather than >> >> > SIGCHLD. CLONE_FD makes it possible for libraries to safely launch and manage >> >> > child processes on behalf of their caller, *without* taking over process-wide >> >> > SIGCHLD handling (either via signal handler or signalfd). >> >> >> >> Hi Josh, >> >> >> >> From the overall description (i.e. I haven't looked at the code yet) >> >> this looks very interesting. However, it seems to cover a lot of the >> >> same ground as the process descriptor feature that was added to FreeBSD >> >> in 9.x/10.x: >> >> https://www.freebsd.org/cgi/man.cgi?query=pdfork&sektion=2 >> > >> > Interesting. >> > >> >> I think it would ideally be nice for a userspace library developer to be >> >> able to do subprocess management (without SIGCHLD) in a similar way >> >> across both platforms, without lots of complicated autoconf shenanigans. >> >> >> >> So could we look at the overlap and seeing if we can come up with >> >> something that covers your requirements and also allows for something >> >> that looks like FreeBSD's process descriptors? >> > >> > Agreed; however, I think it's reasonable to provide appropriate Linux >> > system calls, and then let glibc or libbsd or similar provide the >> > BSD-compatible calls on top of those. I don't think the kernel >> > interface needs to exactly match FreeBSD's, as long as it's a superset >> > of the functionality. >> >> We need to be careful with things like read(2), though. It's hard to >> write a glibc function that makes read(2) do something other than what >> the kernel thinks. Similarly, poll(2) is defined by the kernel. It >> would be really nice to be consistent here. > > It doesn't sound like FreeBSD implements read(2) on the pdfork file > descriptor at all. If it does, yes, we're not going to be able to be > compatible with that. There's an argument that using read(2) for stuff like this is a bad idea. If anyone tried to do this in C++ (or any other OO language): class GenericInterface { public: virtual void DoAction(const char *value, size_t len) = 0; }; class Process : public GenericInterface { public: virtual void DoAction(const char *value, size_t len) = 0; }; void Kill(Process *p) { p->DoAction("kill", 4); }; They'd be re-educated very quickly. This is like duck typing, but taken to a whole new level: *everything* is a duck, and ducks have a grand total of three operations. On the other hand, this seems to be UNIX tradition. It's not as if using echo on pidfds is going to be a common idiom, though. In any event, we should find out what FreeBSD does in response to read(2) on the fd. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
