Re: [PATCH 1/3] mm/page_alloc: effectively disable pcp with CONFIG_SMP=n

[David Hildenbrand (Arm)]

> - * With the UP spinlock implementation, when we spin_lock(&pcp->lock) (for i.e.
> - * a potentially remote cpu drain) and get interrupted by an operation that
> - * attempts pcp_spin_trylock(), we can't rely on the trylock failure due to UP
> - * spinlock assumptions making the trylock a no-op. So we have to turn that
> - * spin_lock() to a spin_lock_irqsave(). This works because on UP there are no
> - * remote cpu's so we can only be locking the only existing local one.
> + * On CONFIG_SMP=n the UP implementation of spin_trylock() never fails and thus
> + * is not compatible with our locking scheme. However we do not need pcp for
> + * scalability in the first place, so just make all the trylocks fail and take
> + * the slow path unconditionally.
>   */
> +#else
> +#define pcp_spin_trylock(ptr)		\
> +		NULL
> +
> +#define pcp_spin_unlock(ptr)		\
> +		BUG_ON(1)

Did you try turning this into a BUILD_BUG() ?
I'd assume that the compiler would optimize-out all dead code and
consequently not trigger the BUILD_BUG.

if (pcp_spin_trylock()) {
	/* dead code */
	pcp_spin_unlock()
}

IIUC, the trylock+unlock is not really split over multiple functions.

-- 
Cheers,

David