64-bit ppc rwsem

[David Miller]

From: Benjamin Herrenschmidt <benh at kernel.crashing.org>
Date: Tue, 24 Aug 2010 08:01:25 +1000

> On Mon, 2010-08-23 at 15:44 +0200, Arnd Bergmann wrote:
>> 
>> * Alpha has an optimization for the uniprocessor case, where the atomic
>> instructions get turned into nonatomic additions. The spinlock based
>> version uses no locks on UP but disables interrupts for reasons I don't
>> understand (nothing running at interrupt time should try to access an rwsem).
>> Should the generic version do the same as Alpha?
> 
> I've seen drivers in the past do trylocks at interrupt time ... tho I
> agree it sucks.

Recently there was a thread where this was declared absolutely illegal.

Maybe it was allowed, or sort-of worked before, and that's why it's
accounted for with IRQ disables in some implementations.  I don't
know.