Messages in this thread | | | Date | Thu, 09 Nov 2006 08:24:22 +0100 | From | Eric Dumazet <> | Subject | Re: [take23 0/5] kevent: Generic event handling mechanism. |
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Davide Libenzi a écrit : > On Thu, 9 Nov 2006, Eric Dumazet wrote: > >> Davide Libenzi a ?crit : >>> I don't care about both ways, but sys_poll() does the same thing epoll does >>> right now, so I would not change epoll behaviour. >>> >> Sure poll() cannot return a partial count, since its return value is : >> >> On success, a positive number is returned, where the number returned is >> the number of structures which have non-zero revents fields (in other >> words, those descriptors with events or errors reported). >> >> poll() is non destructive (it doesnt change any state into kernel). Returning >> EFAULT in case of an error in the very last bit of user area is mandatory. >> >> On the contrary : >> >> epoll_wait() does return a count of transfered events, and update some state >> in kernel (it consume Edge Trigered events : They can be lost forever if not >> reported to user) >> >> So epoll_wait() is much more like read(), that also updates file state in >> kernel (current file position) > > Lost forever means? If there are more processes watching some fd > (external events), they all get their own copy of the events in their own > private epoll fd. It's not that we "steal" things out of the kernel, is > not a 1:1 producer/consumer thing (one producer, 1 queue). It's one > producer, broadcast to all listeners (consumers) thing. The only case > where it'd matter is in the case of multiple threads sharing the same > epoll fd.
In my particular epoll application, the producer is tcp stack, and I have one consumer. If an network event is lost in the EFAULT handling, its lost forever. In any case, my application do provide a correct user area, so this problem is only theorical.
> In general, I'd be more for having the userspace get his own SEGFAULT > instead of letting it go with broken parameters. If I'm coding userspace, > and I'm doing something wrong, I like the kernel to let me know, instead > of trying to fix things for me. > Also, epoll can easily be fixed (add a param to ep_reinject_items() to > re-inject items in case of error/EFAULT) to leave events in the ready-list > and let the EFAULT emerge.
Please dont slow the hot path for a basically "User Error". It's already tested in the transfert function, with two conditional branches for each transfered event.
> Anyone else has opinions about this? > > > > > PS: Next time it'd be great if you Cc: me when posting epoll patches, so > you avoid Andrew the job of doing it.
Yes, but this particular patch was a followup on own kevent Andrew patch.
I have a bunch of patches for epoll I will send to you :)
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