Re: [PATCH] firmware: Do not use WARN_ON(!spin_is_locked())

Theodore Ts'o
2014-08-13T23:32:12Z

On Wed, Aug 13, 2014 at 04:02:01PM +0100, Matt Fleming wrote:
> On Wed, 13 Aug, at 07:37:37AM, Guenter Roeck wrote:
> >
> > WARN_ON and WARN_ON_SMP are unconditional. lockdep_assert_held()
> > is only active if lockdep debugging is enabled. Not knowing the code,
> > nor the reason why the unconditional method was chosen, I prefer
> > to refrain from functional changes and limit myself to bug fixes.
>
> As the author of that code, I feel confident telling you that the
> unconditional method was used because the author is a boob. The code
> isn't so important that we need to unconditionally check the locks, and
> indeed it's possible to run into all sorts of problems when you don't
> use the standard lock-checking functions - the non-SMP crash being a
> good example.

If you want to actually force a BUG_ON if the spinlock is not taken,
even for non-debug kernels, you can use assert_spin_locked(). This
translates to a BUG_ON(!raw_spin_is_locked(x)) on SMP kernels, and a
no-op on UP kernels.

If you're confident in your testing that any problems would be
discovered before you push your patches to linus (and you actually use
lockdep in your testing :-), then lockdep_assert_held() doesn't add
any overhead if !lockdep, and so it might be a better choice for you.

- Ted