linux-kernel - Re: [take24 3/6] kevent: poll/select() notifications.

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Date:	Thu, 9 Nov 2006 10:51:56 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
cc:	David Miller <davem@...emloft.net>,
	Ulrich Drepper <drepper@...hat.com>,
	Andrew Morton <akpm@...l.org>, netdev <netdev@...r.kernel.org>,
	Zach Brown <zach.brown@...cle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Chase Venters <chase.venters@...entec.com>,
	Johann Borck <johann.borck@...sedata.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeff Garzik <jeff@...zik.org>
Subject: Re: [take24 3/6] kevent: poll/select() notifications.

On Thu, 9 Nov 2006, Evgeniy Polyakov wrote:

> +static int kevent_poll_callback(struct kevent *k)
> +{
> +	if (k->event.req_flags & KEVENT_REQ_LAST_CHECK) {
> +		return 1;
> +	} else {
> +		struct file *file = k->st->origin;
> +		unsigned int revents = file->f_op->poll(file, NULL);
> +
> +		k->event.ret_data[0] = revents & k->event.event;
> +		
> +		return (revents & k->event.event);
> +	}
> +}

You need to be careful that file->f_op->poll is not called inside the 
spin_lock_irqsave/spin_lock_irqrestore pair, since (even this came up 
during epoll developemtn days) file->f_op->poll might do a simple 
spin_lock_irq/spin_unlock_irq. This unfortunate constrain forced epoll to 
have a suboptimal double O(R) loop to handle LT events.



- Davide


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