linux-kernel - Re: [PATCH v7 09/12] sysfs: fix deadlock race with module removal

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Message-ID: <YUjfeGuSVVq84iHc@mtj.duckdns.org>
Date:   Mon, 20 Sep 2021 09:22:32 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
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Subject: Re: [PATCH v7 09/12] sysfs: fix deadlock race with module removal

Hello,

On Mon, Sep 20, 2021 at 12:15:22PM -0700, Luis Chamberlain wrote:
> > I find this explanation odd because there's no real equivalent to locking
> > the module (as opposed to try locking) 
> 
> Actually there is, __module_get() but I suspect some of these users are
> probably incorrect and should be be moved to try. The documentation

__module_get() is just getting an extra ref when the caller already
has one (or more). It can't be used to freshly acquire a new
reference. There is no equivalence between the relationship between
try_module_get() and __module_get() and the one between spin_trylock()
and spin_lock().

> Right, the reason I mention the alternative is that we technically don't
> need to use try in this case since during a kernfs op it is implied the
> module will be pinned, but we have further motivations to use a try

I'm confused. If the module is already pinned, why are we getting an
extra reference? Also, I don't understand how this has that much to do
with preventing ddoses. I mean, it does cut down the duration of one
operation but the eventual gating is through whoever acquiring the
initial reference through try_module_get(), which again is the *only*
way to acquire a fresh reference.

Thanks.

-- 
tejun