From: Gleb Natapov Subject: Re: [PATCH-RFC 2/2] eventfd: EFD_STATE flag Date: Tue, 4 Aug 2009 12:17:55 +0300 Message-ID: <20090804091755.GD4764@redhat.com> References: <20090728175538.GC21549@redhat.com> <4A76FDB2.7080706@redhat.com> <20090803151426.GA3630@redhat.com> <4A770260.5000507@redhat.com> <20090803165708.GB3630@redhat.com> <4A77F6EF.8010002@redhat.com> <20090804085406.GA3311@redhat.com> <4A77FCB8.1000205@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "Michael S. Tsirkin" , davidel@xmailserver.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org To: Avi Kivity Return-path: Content-Disposition: inline In-Reply-To: <4A77FCB8.1000205@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Tue, Aug 04, 2009 at 12:17:44PM +0300, Avi Kivity wrote: > On 08/04/2009 11:54 AM, Michael S. Tsirkin wrote: >> On Tue, Aug 04, 2009 at 11:53:03AM +0300, Avi Kivity wrote: >> >>> On 08/03/2009 07:57 PM, Michael S. Tsirkin wrote: >>> >>>>> Why not do it at the point of the write? >>>>> >>>>> if (value != ctx->count) { >>>>> ctx->count = value; >>>>> wake_things_up(); >>>>> } >>>>> >>>>> >>>> What if write comes before read? >>>> >>>> >>> The read will get the new value. >>> >> >> Yes :) But how does read know it should not block? >> > > If a different read comes after the write but after our read, it will > have transferred the value, resulting in the same situation. > > I think reads should never block with a state based mechanism. > Reader may want to poll for the status change. -- Gleb.