[PATCH] synchronize_irq needs a barrier

[Benjamin Herrenschmidt]

> 
> In general, I tend to think that for this function to make any sense
> (that is, to synchronize anything at all), it needs a barrier or you are
> just making a decision based on a totally random value of desc->status
> since it can have been re-ordered, speculatively loaded, pre-fetched,
> whatever'ed... :-).

Take a real life example:

drivers/message/fusion/mptbase.c

	/* Disable interrupts! */
	CHIPREG_WRITE32(&ioc->chip->IntMask, 0xFFFFFFFF);

	ioc->active = 0;
	synchronize_irq(pdev->irq);

And we aren't in a spinlock here. 

That's just a random example grepped.... I think I see a few more. Then,
some drivers like tg3 actually do an smp_mb() before calling
synchronize_irq(). But then, some don't.

I think trying to have all drivers be correct here is asking for
trouble, we'd rather have synchronize_irq() be uber-safe. It's not like
it was used in hot path anyway.

Ben.