Re: [take3 1/4] kevent: Core files.

Evgeniy Polyakov
2006-08-03T15:22:37Z

On Thu, Aug 03, 2006 at 05:11:58PM +0200, Eric Dumazet (dada1@cosmosbay.com) wrote:
> On Thursday 03 August 2006 16:55, Evgeniy Polyakov wrote:
> > On Thu, Aug 03, 2006 at 04:40:34PM +0200, Eric Dumazet (dada1@cosmosbay.com)
> wrote:
> > > > + mutex_lock(&u->ctl_mutex);
> > > > + while (num < max_nr && ((k = kqueue_dequeue_ready(u)) != NULL)) {
> > > > + if (copy_to_user(buf + num*sizeof(struct ukevent),
> > > > + &k->event, sizeof(struct ukevent))) {
> > > > + cerr = -EINVAL;
> > > > + break;
> > > > + }
> > >
> > > It seems quite wrong to hold ctl_mutex while doing a copy_to_user() (of
> > > possibly a large amount of data) : A thread can sleep on a page fault and
> > > other threads cannot make progress.
> >
> > I would not call that wrong - system prevents some threads from removing
> > kevents which are counted to be transfered to the userspace, i.e. when
> > dequeuing was awakened and it had seen some events it is possible, that
> > when it will dequeue them part will be removed by other thread, so I
> > prevent this.
>
> Hum, "wrong" was maybe not the good word.... but kqueue_dequeue_ready() uses a
> spinlock (ready_lock) to protect ready_list. One particular struct kevent is
> given to one thread, one at a time.

I mean that wait_event logic will see that there are requested number of
events, and when it starts to get them, it is possible that there will
be no events at all.

> If you look at fs/eventpoll.c, you can see how carefull is ep_send_events() so
> that multiple threads can in the same time transfer different items to user
> memory.

It is done under the same logic under ep->sem semaphore, which is being
held for del and read operations.
Or do you mean to have rw semahore instead of mutex here?

> In a model where several threads are servicing events collected by a single
> point (epoll, or kevent), this is important to not block all threads because
> of a single thread waiting a swapin (trigered by copy_to_user() )

> Eric

--
Evgeniy Polyakov