Re: [PATCH 1/5] Swap Migration V5: LRU operations

[Andrew Morton <akpm@osdl.org>]

Christoph Lameter <clameter@engr.sgi.com> wrote:
>
> On Tue, 15 Nov 2005, Andrew Morton wrote:
> 
> > But lru_add_drain_per_cpu() will be called from interrupt context: the IPI
> > handler.
> 
> Ahh.. thought you meant the lru_add_drain run on the local processor.
>  
> > I'm asking whether it is safe for the IPI handler to reenable interupts on
> > all architectures.  It might be so, but I don't recall ever having seen it
> > discussed, nor have I seen code which does it.
> 
> smp_call_function is also used by the slab allocator to drain the 
> pages. All the spinlocks in there and those of the page allocator (called 
> for freeing pages) use spin_lock_irqsave. Why is this not used for 
> lru_add_drain() and friends?

It's a microoptimisation - lru_add_drain() is always called with local irqs
enabled, so no need for irqsave.

I don't think spin_lock_irqsave() is notably more expensive than
spin_lock_irq() - the cost is in the irq disabling and in the atomic
operation.

> Maybe we need to start a new thread so that others see it?

Spose so.  If we cannot convince ourselves that local_irq_enable() in an
ipi handler is safe, we need to convert any called functions to use
irqsave.