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Date:   Mon, 20 Sep 2021 17:03:27 -0700
From:   Luis Chamberlain <mcgrof@kernel.org>
To:     Dan Williams <dan.j.williams@intel.com>
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Subject: Re: [PATCH v7 09/12] sysfs: fix deadlock race with module removal
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On Mon, Sep 20, 2021 at 02:55:10PM -0700, Dan Williams wrote:
> On Mon, Sep 20, 2021 at 2:17 PM Luis Chamberlain <mcgrof@kernel.org> wrote:
> >
> > On Mon, Sep 20, 2021 at 01:52:21PM -0700, Dan Williams wrote:
> > > On Fri, Sep 17, 2021 at 10:05 PM Luis Chamberlain <mcgrof@kernel.org> wrote:
> > > > This deadlock was first reported with the zram driver, however the live
> > > > patching folks have acknowledged they have observed this as well with
> > > > live patching, when a live patch is removed. I was then able to
> > > > reproduce easily by creating a dedicated selftests.
> > > >
> > > > A sketch of how this can happen follows:
> > > >
> > > > CPU A                              CPU B
> > > >                                    whatever_store()
> > > > module_unload
> > > >   mutex_lock(foo)
> > > >                                    mutex_lock(foo)
> > > >    del_gendisk(zram->disk);
> > > >      device_del()
> > > >        device_remove_groups()
> > >
> > > This flow seems possible to trigger with:
> > >
> > >    echo $dev > /sys/bus/$bus/drivers/$driver/unbind
> > >
> > > I am missing why module pinning
> >
> > The aspect of try_module_get() which comes to value to prevent the
> > deadlock is it ensures kernfs ops do not run once exit is on the way.
> >
> > > is part of the solution when it's the
> > > device_del() path that is racing?
> >
> > But its not, the device_del() path will yield until the kernfs op
> > completes. It is fine to wait there.
> >
> > The deadlock happens if a module exit routine uses a lock which is
> > also used on a sysfs op. If the lock was first held by module exit,
> > and module exit is waiting for the kernfs op to complete, and the
> > kernfs op is waiting to hold the same lock then the exit will wait
> > forever.
> >
> > > Module removal is just a more coarse
> > > grained way to trigger unbind => device_del().
> >
> > Right, but the device_del() path is not sharing a lock with the sysfs op.
> 
> The deadlock in the example comes from holding a lock over
> device_del() [...]

No sorry, that is my mistake not making it clear that the mutex held
in the example is on module exit. Or any lock for that matter. That is
these locks are driver specific.

> > > Isn't the above a bug
> > > in the driver, not missing synchronization in kernfs?
> >
> > We can certainly take the position as an alternative:
> >
> >   "thou shalt not use a lock on exit which is also used on a syfs op"
> >
> > However that seems counter intuitive, specially if we can resolve the
> > issue easily with a try_module_get().
> 
> Again, I don't see how try_module_get() can affect the ABBA failure
> case of holding a lock over device_del() that is also held inside
> sysfs op.

It is not device_del(), it is on module exit. Sorry for this not being
clear before. I'll fix the commit log to make it clearer. The subject
at least was clear but I think the example could be clearer.

  Luis