Messages in this thread | | | Date | Sat, 12 Jul 2003 18:11:47 -0500 | From | Eric Varsanyi <> | Subject | Re: [Patch][RFC] epoll and half closed TCP connections |
| |
> > I guess my only argument would be that edge triggered mode isn't really > > workable with TCP connections if there's no way to solve the ambiguity > > between EOF and no data in buffer (at least w/o an extra syscall). I just > > realized that the race you mention in the man page (reading data from > > the 'next' event that hasn't been polled into user mode yet) will lead to > > the same issue: how do you know if you got this event because you consumed > > the data on the previous interrupt or if this is an EOF condition. > > (Sorry, I missed this) > You can work that out very easily. When your read/write returns a lower > number of bytes, it means that it is time to stop processing this fd. If > events happened meanwhile, you will get them at the next epoll_wait(). If > not, the next time they'll happen. There's no blind spot if you follow > this simple rule, and you do not even have the extra syscall with EAGAIN.
The scenario that I think is still uncovered (edge trigger only):
User Kernel -------- ---------- Read data added to socket
Socket posts read event to epfd
epoll_wait() Event cleared from epfd, EPOLLIN returned to user
more read data added to socket
Socket posts a new read event to epfd
read() until short read with EAGAIN all data read from socket
epoll_wait() returns another EPOLLIN for socket and clears it from epfd
read(), returns 0 right away socket buffer is empty
This is your 'false positive' case in the epoll(4) man page.
How does the app tell the 0 read here from a read EOF coming from the remote?
If it assumes this is a false positive and there was also an EOF indication, the EOF will be lost; if it assumes it an EOF the connection will be prematurely terminated.
-Eric - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |