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Date:   Fri, 11 Feb 2022 12:09:58 +0100
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Abhinav Kumar <quic_abhinavk@quicinc.com>
Cc:     Johannes Berg <johannes@sipsolutions.net>,
        linux-kernel@vger.kernel.org, rafael@kernel.org,
        linux-arm-msm@vger.kernel.org, dri-devel@lists.freedesktop.org,
        swboyd@chromium.org, khsieh@codeaurora.org, nganji@codeaurora.org,
        seanpaul@chromium.org, dmitry.baryshkov@linaro.org,
        aravindh@codeaurora.org, freedreno@lists.freedesktop.org
Subject: Re: [PATCH] devcoredump: increase the device delete timeout to 10
 mins
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On Tue, Feb 08, 2022 at 05:55:18PM -0800, Abhinav Kumar wrote:
> Hi Johannes
> 
> On 2/8/2022 1:54 PM, Johannes Berg wrote:
> > On Tue, 2022-02-08 at 13:40 -0800, Abhinav Kumar wrote:
> > > > 
> > > I am checking what usermode sees and will get back ( I didnt see an
> > > error do most likely it was EOF ). I didnt follow the second part.
> > 
> > I think probably it got -ENODEV, looking at kernfs_file_read_iter().
> > 
> > > If the file descriptor read returns EOF, even if we consider them
> > > separate how will it resolve this issue?
> > > 
> > > My earlier questions were related to fixing it in devcoredump to detect
> > > and fix it there. Are you suggesting to fix in usermode instead? How?
> > > 
> > 
> > Yeah, no, you cannot fix it in userspace.
> > 
> > But I just followed the rabbit hole down kernfs and all, and it looks
> > like indeed the read would be cut short with -ENODEV, sorry.
> > 
> > It doesn't look like there's good API for this, but it seems at least
> > from the underlying kernfs POV it should be possible to get_device() in
> > open and put_device() in release, so that the device sticks around while
> > somebody has the file open? It's entirely virtual, so this should be OK?
> > 
> > johannes
> 
> Are you suggesting something like below?
> 
> diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
> index 42dcf96..14203d0 100644
> --- a/fs/sysfs/file.c
> +++ b/fs/sysfs/file.c
> @@ -32,6 +32,22 @@ static const struct sysfs_ops *sysfs_file_ops(struct
> kernfs_node *kn)
>         return kobj->ktype ? kobj->ktype->sysfs_ops : NULL;
>  }
> 
> +static int sysfs_kf_open(struct kernfs_open_file *of)
> +{
> +       struct kobject *kobj = of->kn->parent->priv;
> +       struct device *dev = kobj_to_dev(kobj);
> +
> +       get_device(dev);
> +}
> +
> +static void sysfs_kf_release(struct kernfs_open_file *of)
> +{
> +       struct kobject *kobj = of->kn->parent->priv;
> +       struct device *dev = kobj_to_dev(kobj);
> +
> +       put_device(dev);
> +}


That obviously does not work as not everything in sysfs is a struct
device :(