On Fri, Feb 29, 2008 at 8:19 PM, Davide Libenzi <davidel@xxxxxxxxxxxxxxx> wrote: > On Fri, 29 Feb 2008, Michael Kerrisk wrote: > > > As I think is clear, I've only given it very limited thought ;-). > > > > The point is that the existing implementation actually supports > > "different *processes* sharing a single epoll fd and doing > > epoll_wait() over it", but the semantics are unintuitive. It may be > > that the existing implementation was the best way of doing things. > > But when I see the strange corner cases in the semantics, I can't help > > but wonder (way too late), whether there might have been some other > > way of implementing things that led to more intuitive semantics. > > Oh boy. The fact that you can have an epoll fd cross the fork boundary, > does not mean that any indiscriminate use of it leads to sane results: I ddidn't mean that it did. Certainly in the current implementation it there will insane situations ;-). > efd = epoll_create(); > fork(); > pipe(fds); > epoll_ctl(efd, ADD, fds[0]); > epoll_wait(); ???? > ... > pipe(fds); > epoll_ctl(efd, ADD, fds[0]); > epoll_wait(); ???? > > > It is *NOT* a matter of semantics. Of course -- but I don't think I suggested that I disagree on this. > > > If the next question is "But then why we made the epoll fd inheritable?", > > > the answer is, because it makes sense in many cases for a parent to hand > > > over an fd set to a child. > > > > Fair enough. > > > > So here's an idea about how things might alternatively have been done: > > > > a) The key for epoll entries could have been [file *, fd, PID] > > > > b) an epoll_wait() only returns events for fds where the PID maps that > > of the caller. > > > > c) a close of a file descriptor removes the corresponding [file *, > > fd, PID] from the epoll set. > > > > d) when a fork() is done, then the epoll set has a new set of keys > > added. These are duplicates of the [file *, fd, PID] entries for the > > parent, but with the PID of the child substituted into the new keys. > > Say the parent had PID 1000, and the child has PID 2000. If the epoll > > set initially contained: > > > > [X, 3, 1000] > > [Y, 4, 1000] > > > > then after fork() we'd have: > > > > [X, 3, 1000] > > [Y, 4, 1000] > > [X, 3, 2000] > > [Y, 4, 2000] > > > > There is of course room for debate about the efficiency of this > > approach, I suppose. > > There sure is :) Okay -- but I suspect it could have been made fairly efficient. > > You said elsewhere: > > > > [[ > > That'd mean placing an eventpoll custom hook into sys_close(). Looks very > > bad to me, and probably will look even worse to other kernel folks. > > Is not much a performance issue (a check to see if a file* is an eventpoll > > file is as easy as comparing the f_op pointer), but a design/style issue. > > ]] > > > > But that wasn't very clear to me actually. I note that filp_close() > > already has special case handling for dnotify (R.I.P.) and fcntl() > > )aka POSIX) file locks, so there was already precedent for a custom > > hook, AFAICS, and epoll is at least as worthy of special treatment as > > either of those cases. > > I guess that over the time, Al became software WRT junk going there :) Sorry -- I don't understand that last sentence? -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
