linux-kernel - Re: 64-bit ppc rwsem

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Message-ID: <1282600885.22370.453.camel@pasglop>
Date:	Tue, 24 Aug 2010 08:01:25 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linuxppc-dev@...ts.ozlabs.org, David Miller <davem@...emloft.net>,
	torvalds@...ux-foundation.org, paulus@...ibm.com,
	linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: 64-bit ppc rwsem

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.

> * Is there any architecture that would still benefit from having a separate
> rwsem implementation? AFAICT all the remaining ones are just variations of
> the same concept of using cmpxchg (or xadd in case of x86), which is what
> atomics typically end up doing anyway.

It depends how sensitive rwsems are. 

The "generic" variant based on atomic's and cmpxchg on powerpc is
sub-optimal in the sense that it has stronger memory barriers that would
be necessary (atomic_inc_return for example has both acquire and
release).

But that vs. one more pile of inline asm, we decided it wasn't hot
enough a spot for us to care back then.

Cheers,
Ben.

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