From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCHv2 1/3] eventfd: allow atomic read and waitqueue remove Date: Thu, 21 Jan 2010 19:57:22 +0200 Message-ID: <4B589582.9030300@redhat.com> References: <20100121162648.GA16458@redhat.com> <4B588B29.2050100@redhat.com> <20100121172336.GA16707@redhat.com> <4B588FCE.7030803@redhat.com> <20100121173259.GC16707@redhat.com> <4B58933C.4090209@redhat.com> <20100121174538.GG16707@redhat.com> <4B589532.9070408@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Davide Libenzi , mtosatti@redhat.com, kvm@vger.kernel.org, Linux Kernel Mailing List To: "Michael S. Tsirkin" Return-path: Received: from mx1.redhat.com ([209.132.183.28]:45197 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754620Ab0AUR50 (ORCPT ); Thu, 21 Jan 2010 12:57:26 -0500 In-Reply-To: <4B589532.9070408@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 01/21/2010 07:56 PM, Avi Kivity wrote: > On 01/21/2010 07:45 PM, Michael S. Tsirkin wrote: >> >>> But you're in process context. An eventfd never blocks. >> Yes it blocks if counter is 0. And we don't know >> it's not 0 unless we read :) catch-22. > > Ah yes, I forgot. > Well, you can poll it and then read it... this introduces a new race (if userspace does a read in parallel) but it's limited to kvm and buggy userspace. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.