From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757643AbcJZJR0 (ORCPT ); Wed, 26 Oct 2016 05:17:26 -0400 Received: from mail-wm0-f65.google.com ([74.125.82.65]:33950 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752363AbcJZJRX (ORCPT ); Wed, 26 Oct 2016 05:17:23 -0400 MIME-Version: 1.0 In-Reply-To: <1476274060-1785-1-git-send-email-tomasz.majchrzak@intel.com> References: <1476274060-1785-1-git-send-email-tomasz.majchrzak@intel.com> From: Miklos Szeredi Date: Wed, 26 Oct 2016 11:17:13 +0200 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2][RESEND] seq_file: don't set read position for invalid iterator To: Tomasz Majchrzak Cc: LKML , dan.j.williams@intel.com, Al Viro , aleksey.obitotskiy@intel.com, pawel.baldysiak@intel.com, artur.paszkiewicz@intel.com, maksymilian.kunt@intel.com Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 12, 2016 at 2:07 PM, Tomasz Majchrzak wrote: > If kernfs file is empty on a first read, successive read operations > using the same file descriptor will return no data, even when data is > available. Default kernfs 'seq_next' implementation advances iterator > position even when next object is not there. Kernfs 'seq_start' for > following requests will not return iterator as position is already on > the second object. > > This bug doesn't allow to monitor badblocks sysfs files from MD raid. > They are initially empty but if data appears at some stage, userspace is > not able to read it. It doesn't affect any released applications but it > is necessary for upcoming bad block support for external metadata in MD > raid. What is the expectation from the userspace application? Should seq_file support "tail -f" as more data is added to the file? AFAICS this patch doesn't address that generally, just the empty->nonempty transition (with the single record case). Why does userspace app not do open+read+close each time it polls the badblocks file? Thanks, Miklos > > Signed-off-by: Tomasz Majchrzak > Reviewed-by: Dan Williams > --- > fs/seq_file.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > diff --git a/fs/seq_file.c b/fs/seq_file.c > index 6dc4296..74197f4 100644 > --- a/fs/seq_file.c > +++ b/fs/seq_file.c > @@ -235,7 +235,7 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos) > p = m->op->start(m, &pos); > while (1) { > err = PTR_ERR(p); > - if (!p || IS_ERR(p)) > + if (IS_ERR_OR_NULL(p)) > break; > err = m->op->show(m, p); > if (err < 0) > @@ -244,7 +244,8 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos) > m->count = 0; > if (unlikely(!m->count)) { > p = m->op->next(m, p, &pos); > - m->index = pos; > + if (!IS_ERR_OR_NULL(p)) > + m->index = pos; > continue; > } > if (m->count < m->size) > -- > 1.8.3.1 >