Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754751AbWKITOT (ORCPT ); Thu, 9 Nov 2006 14:14:19 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754765AbWKITOT (ORCPT ); Thu, 9 Nov 2006 14:14:19 -0500 Received: from relay.2ka.mipt.ru ([194.85.82.65]:6368 "EHLO 2ka.mipt.ru") by vger.kernel.org with ESMTP id S1754363AbWKITOR (ORCPT ); Thu, 9 Nov 2006 14:14:17 -0500 Date: Thu, 9 Nov 2006 22:10:36 +0300 From: Evgeniy Polyakov To: Davide Libenzi Cc: David Miller , Ulrich Drepper , Andrew Morton , netdev , Zach Brown , Christoph Hellwig , Chase Venters , Johann Borck , Linux Kernel Mailing List , Jeff Garzik Subject: Re: [take24 3/6] kevent: poll/select() notifications. Message-ID: <20061109191036.GA30138@2ka.mipt.ru> References: <11630606373650@2ka.mipt.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=koi8-r Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.9i X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-1.7.5 (2ka.mipt.ru [0.0.0.0]); Thu, 09 Nov 2006 22:10:38 +0300 (MSK) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1940 Lines: 46 On Thu, Nov 09, 2006 at 10:51:56AM -0800, Davide Libenzi (davidel@xmailserver.org) wrote: > 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. It is tricky - users call wake_up() from any context, which in turn ends up calling kevent_storage_ready(), which calls kevent_poll_callback() with KEVENT_REQ_LAST_CHECK bit set, which becomes almost empty call in fast path. Since callback returns 1, kevent will be queued into ready queue, which is processed on behalf of syscalls - in that case kevent will check the flag and since KEVENT_REQ_LAST_CHECK is set, will call callback again to check if kevent is correctly marked, but already without that flag (it happens in syscall context, i.e. process context without any locks held), so callback calls ->poll(), which can sleep, but it is safe. If ->poll() returns 'ready' value, kevent is transfers data into userspace, otherwise it is 'requeued' (just removed from ready queue). > - Davide > -- Evgeniy Polyakov - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/