Re: [PATCH] devcoredump: increase the device delete timeout to 10 mins

[Abhinav Kumar <quic_abhinavk@quicinc.com>]

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);
+}
+
  /*
   * Reads on sysfs are handled through seq_file, which takes care of hairy
   * details like buffering and seeking.  The following function pipes
@@ -211,6 +227,8 @@ static const struct kernfs_ops sysfs_file_kfops_wo = {
  };

  static const struct kernfs_ops sysfs_file_kfops_rw = {
+       .open       = sysfs_kf_open;
+       .release    = sysfs_kf_release;
         .seq_show       = sysfs_kf_seq_show,
         .write          = sysfs_kf_write,
  };

If so, dont you think this will be a more intrusive change just for the 
sake of devcoredump? Any other way to keep the changes limited to 
devcoredump?

Thanks

Abhinav