From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:34149) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aYCPl-0008Ua-Jf for qemu-devel@nongnu.org; Tue, 23 Feb 2016 07:50:06 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aYCPk-00015y-TD for qemu-devel@nongnu.org; Tue, 23 Feb 2016 07:50:05 -0500 Date: Tue, 23 Feb 2016 20:49:56 +0800 From: Fam Zheng Message-ID: <20160223124956.GA26812@ad.usersys.redhat.com> References: <1455638000-18051-1-git-send-email-pbonzini@redhat.com> <20160217025722.GC30207@ad.usersys.redhat.com> <56C4595D.1020206@redhat.com> <20160223055704.GC19080@ad.usersys.redhat.com> <56CC37EB.7050901@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <56CC37EB.7050901@redhat.com> Subject: Re: [Qemu-devel] [PATCH] qed: fix bdrv_qed_drain List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: stefanha@redhat.com, qemu-devel@nongnu.org, qemu-block@nongnu.org, qemu-stable@nongnu.org On Tue, 02/23 11:43, Paolo Bonzini wrote: > > > On 23/02/2016 06:57, Fam Zheng wrote: > >>>> + qed_cancel_need_check_timer(s); > >>>> + qed_need_check_timer_cb(s); > >>>> + } > >>> > >>> What if an allocating write is queued (the else branch case)? Its completion > >>> will be in bdrv_drain and it could arm the need_check_timer which is wrong. > >>> > >>> We need to drain the allocating_write_reqs queue before checking the timer. > >> > >> You're right, but how? That's what bdrv_drain(bs) does, it's a > >> chicken-and-egg problem. > > > > Maybe use an aio_poll loop before the if? > > That would not change the fact that you're reimplementing bdrv_drain > inside bdrv_qed_drain. > But it fulfills the contract of .bdrv_drain. This is the easy way, the hard way would be iterating through the allocating_write_reqs list and process reqs one by one synchronously, which still involves aio_poll indirectly. > Perhaps for now it's simplest to just remove the QED .bdrv_drain > callback, if you think this patch is not a good stopgap measure to avoid > the segmentation faults. OK, I'm fine with this as a stopgap measure. > > Once the bdrv_drain rework is in, we can move the callback _after_ I/O > is drained on bs and before it is drained on bs->file->bs. Sounds good. Fam