kqueue oddity

[Václav Haisman]

Dan Nelson wrote:
> In the last episode (Apr 01), Vclav Haisman said:
>> Hi,
>> I have a little problem with kqueue/kevent. I am getting somewhat
>> strange events from kevent(). The struct kevent's contents looks like this:
>>
>> kevent {.ident =  1, .filter = 0xfffffffe, .flags = 0x0001, 
>>   .fflags = 0x00000000, .data = 0x000084c6}
>>
>> .ident == 1 is stdout.
>> .filter == 0x fffffffe is -2 is EVFILT_WRITE which makes sense for stdout.
>> .fflags is empty, nothing out of ordinary for EVFILT_WRITE.
>> And here comes the oddity: .flags == 0x0001 means EV_ADD which totally
>> doesn't make sense to me and neither does according to man kqueue. The
>> code that exhibits this behaviour is io_loop_handler_run() in
>> ioloop-kqueue.c of Dovecot IMAP server.
> 
> It's a kqueue bug, but a minor one.  The problem is that the same
> "flags" field is used to pass "actions" from the client, and return
> status from the kernel.  When you call kqueue with EV_ADD, the kernel
> never clears the flags when creating the internal object, so it's
> always visible when events fire.  This can get annoying when trussing
> kqueue-using programs because very single event has the EV_ADD flag :)
> Apply the following diff to have the kernel strip EV_ADD:
>  
> Index: kern_event.c
> ===================================================================
> RCS file: /home/ncvs/src/sys/kern/kern_event.c,v
> retrieving revision 1.79.2.9
> diff -u -p -r1.79.2.9 kern_event.c
> --- kern_event.c	29 Jun 2005 18:08:31 -0000	1.79.2.9
> +++ kern_event.c	7 Jul 2005 18:51:29 -0000
> @@ -864,6 +864,7 @@ findkn:
>  			kev->fflags = 0;
>  			kev->data = 0;
>  			kn->kn_kevent = *kev;
> +			kn->kn_kevent.flags &= ~EV_ADD; 
>  			kn->kn_status = KN_INFLUX|KN_DETACHED;
>  
>  			error = knote_attach(kn, kq);
> 
> 
Thanks. Why isn't this fix applied? From the patch's time stamp it seems
like it is known for quite some time now.


Vaclav Haisman

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