From: Miklos Szeredi <miklos@szeredi.hu>
To: Tomasz Majchrzak <tomasz.majchrzak@intel.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
dan.j.williams@intel.com, Al Viro <viro@zeniv.linux.org.uk>,
aleksey.obitotskiy@intel.com, pawel.baldysiak@intel.com,
artur.paszkiewicz@intel.com, maksymilian.kunt@intel.com
Subject: Re: [PATCH v2][RESEND] seq_file: don't set read position for invalid iterator
Date: Wed, 26 Oct 2016 11:17:13 +0200 [thread overview]
Message-ID: <CAELBmZAFPHu-SaMTJq0wH-2_OxUPVwfpRSxdMFW93po1SybRXA@mail.gmail.com> (raw)
In-Reply-To: <1476274060-1785-1-git-send-email-tomasz.majchrzak@intel.com>
On Wed, Oct 12, 2016 at 2:07 PM, Tomasz Majchrzak
<tomasz.majchrzak@intel.com> 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 <tomasz.majchrzak@intel.com>
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> ---
> 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
>
next prev parent reply other threads:[~2016-10-26 9:17 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-10-12 12:07 [PATCH v2][RESEND] seq_file: don't set read position for invalid iterator Tomasz Majchrzak
2016-10-26 9:17 ` Miklos Szeredi [this message]
2016-10-31 9:32 ` Tomasz Majchrzak
2016-11-02 9:11 ` Miklos Szeredi
2016-11-24 15:23 ` Tomasz Majchrzak
2016-11-24 15:39 ` Miklos Szeredi
2016-11-28 15:11 ` [PATCH v3] seq_file: reset iterator to first record for zero offset Tomasz Majchrzak
2016-11-29 9:58 ` Miklos Szeredi
2016-11-29 14:18 ` [PATCH v4] " Tomasz Majchrzak
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