From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <dada1@cosmosbay.com>
Subject: Re: [take23 0/5] kevent: Generic event handling mechanism.
Date: Thu, 09 Nov 2006 00:07:37 +0100
Message-ID: <45526339.3040506@cosmosbay.com>
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Cc: Andrew Morton <akpm@osdl.org>,
	Evgeniy Polyakov <johnpol@2ka.mipt.ru>,
	David Miller <davem@davemloft.net>,
	Ulrich Drepper <drepper@redhat.com>,
	netdev <netdev@vger.kernel.org>,
	Zach Brown <zach.brown@oracle.com>,
	Christoph Hellwig <hch@infradead.org>,
	Chase Venters <chase.venters@clientec.com>,
	Johann Borck <johann.borck@densedata.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Jeff Garzik <jeff@garzik.org>
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In-reply-to: <Pine.LNX.4.64.0611081442420.23154@alien.or.mcafeemobile.com>
To: Davide Libenzi <davidel@xmailserver.org>
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Davide Libenzi a =E9crit :
> On Wed, 8 Nov 2006, Andrew Morton wrote:
>=20
>> On Wed, 8 Nov 2006 15:51:13 +0100
>> Eric Dumazet <dada1@cosmosbay.com> wrote:
>>
>>> [PATCH] eventpoll : In case a fault occurs during copy_to_user(), w=
e should=20
>>> report the count of events that were successfully copied into user =
space,=20
>>> instead of EFAULT. That would be consistent with behavior of read/w=
rite()=20
>>> syscalls for example.
>>>
>>> Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
>>>
>>>
>>>
>>> [eventpoll.patch  text/plain (424B)]
>>> --- linux/fs/eventpoll.c	2006-11-08 15:37:36.000000000 +0100
>>> +++ linux/fs/eventpoll.c	2006-11-08 15:38:31.000000000 +0100
>>> @@ -1447,7 +1447,7 @@
>>>  				       &events[eventcnt].events) ||
>>>  			    __put_user(epi->event.data,
>>>  				       &events[eventcnt].data))
>>> -				return -EFAULT;
>>> +				return eventcnt ? eventcnt : -EFAULT;
>>>  			if (epi->event.events & EPOLLONESHOT)
>>>  				epi->event.events &=3D EP_PRIVATE_BITS;
>>>  			eventcnt++;
>>>
>> Definitely a better interface, but I wonder if it's too late to chan=
ge it.
>>
>> An app which does
>>
>> 	if (epoll_wait(...) =3D=3D -1)
>> 		barf(errno);
>> 	else
>> 		assume_all_events_were_received();
>>
>> will now do the wrong thing.
>>
>> otoh, such an applciation basically _has_ to use the epoll_wait()
>> return value to work out how many events it received, so maybe it's =
OK...
>=20
> I don't care about both ways, but sys_poll() does the same thing epol=
l=20
> does right now, so I would not change epoll behaviour.
>=20

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). Ret=
urning=20
EFAULT in case of an error in the very last bit of user area is mandato=
ry.

On the contrary :

epoll_wait() does return a count of transfered events, and update some =
state=20
in kernel (it consume Edge Trigered events : They can be lost forever i=
f not=20
reported to user)

So epoll_wait() is much more like read(), that also updates file state =
in=20
kernel (current file position)