Re: [PATCH] sysfs: driver core: Fix glue dir race condition

[Greg KH]

On Thu, Nov 06, 2014 at 11:55:47AM -0500, Tejun Heo wrote:
> Maybe "fix glue dir race condition by not removing them" is a better
> title?
> 
> On Thu, Nov 06, 2014 at 04:16:38PM +0800, Yijing Wang wrote:
> > There is a race condition when removing glue directory.
> > It can be reproduced in following test:
> > 
> > path 1: Add first child device
> > device_add()
> > 	get_device_parent()
> > 		/*find parent from glue_dirs.list*/
> > 		list_for_each_entry(k, &dev->class->p->glue_dirs.list, entry)
> > 			if (k->parent == parent_kobj) {
> > 				kobj = kobject_get(k);
> > 				break;
> > 			}
> > 		....
> > 		class_dir_create_and_add()
> > 
> > path2: Remove last child device under glue dir
> > device_del()
> > 	cleanup_device_parent()
> > 		cleanup_glue_dir()
> > 			kobject_put(glue_dir);
> > 
> > If path2 has been called cleanup_glue_dir(), but not
> > call kobject_put(glue_dir), the glue dir is still
> > in parent's kset list. Meanwhile, path1 find the glue
> > dir from the glue_dirs.list. Path2 may release glue dir
> > before path1 call kobject_get(). So kernel will report
> > the warning and bug_on.
> > 
> > This fix keep glue dir around once it created suggested
> > by Tejun Heo.
> 
> I think you prolly want to explain why this is okay / desired.
> e.g. list how the glue dir is used and how many of them are there and
> explain that there's no real benefit in removing them.

I'd really _like_ to remove them if at all possible, as if there isn't
any "children" in the subdirectory, there shouldn't be a need for that
directory to be there.

This seems to be the "classic" problem we have of a kref in a list that
can be found while the last instance could be removed at the same time.
I hate to just throw another lock at the problem, but wouldn't a lock to
protect the list of glue_dirs be the answer here?

thanks,

greg k-h
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