[dm-devel] [PATCH] kobject: provide kobject_put_wait to fix module unload race

[Al Viro]

On Sat, Jan 04, 2014 at 10:16:20AM -0800, Greg Kroah-Hartman wrote:

> > I came up with a simpler patch to achieve the same purpose - this patch 
> > makes fixing the drivers easy - the driver is fixed just by replacing 
> > "kobject_put" with "kobject_put_wait" in the unload routine.
> 
> No, that's not ok at all.

Agreed - all it takes is one cargo-culter who religoiusly does such
conversion and drops a ref to parent before that to child.

> > However, this pattern is buggy with respect to modules. The release method
> > is placed in the driver's module. When the driver exits, the module
> > reference count is zero, thus the module may be freed. However, there may
> > still be references to the kobject. If the module is unloaded and then the
> > release method is called, a crash happens.
> 
> Yes, module unloading while a kobject is still "active" is not a good
> thing, what modules do you have that cause this problem?  Why not just
> grab the module reference in your kobject if you need this type of
> protection?  It's not the kobject's code fault that this issue is there,
> or that we now have a "delayed release" function to expose this type of
> thing, it's the user of the kobject.
> 
> Please fix the broken users of the kobject first.

<snide> Are you saying that there is another kind? </snide>

When would you grab that reference to module?  More to the point, when
would you *drop* it?  Doing so from module_exit is not going to work,
obviously...