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

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Message-ID: <20060803145553.GA12915@2ka.mipt.ru>
Date:	Thu, 3 Aug 2006 18:55:57 +0400
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Ulrich Drepper <drepper@...hat.com>,
	netdev <netdev@...r.kernel.org>,
	Zach Brown <zach.brown@...cle.com>
Subject: Re: [take3 1/4] kevent: Core files.

On Thu, Aug 03, 2006 at 04:40:34PM +0200, Eric Dumazet (dada1@...mosbay.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.

> Eric

-- 
	Evgeniy Polyakov
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