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



On Sat, Jan 04, 2014 at 06:34:03PM +0000, Al Viro wrote:
> 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...

You normally have subsystem core module that does handle release of its
objects and users of said objects so it is usually OK for objects to
outlive the users, you just need to make sure the core stays around.

In input we grab module reference to input core when we allocate input
device and drop it when input device is freed. This way we can be sure
that input core stays around until all input devices are gone. The same
for serio.

Thanks. 

-- 
Dmitry


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