From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: linuxppc-dev@lists.ozlabs.org, David Miller <davem@davemloft.net>,
torvalds@linux-foundation.org, paulus@au.ibm.com,
linux-kernel@vger.kernel.org, sparclinux@vger.kernel.org,
akpm@linux-foundation.org
Subject: Re: 64-bit ppc rwsem
Date: Tue, 24 Aug 2010 08:01:25 +1000 [thread overview]
Message-ID: <1282600885.22370.453.camel@pasglop> (raw)
In-Reply-To: <201008231544.16422.arnd@arndb.de>
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.
next prev parent reply other threads:[~2010-08-23 22:02 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-08-18 1:03 [GIT] Sparc David Miller
2010-08-18 1:31 ` Linus Torvalds
2010-08-18 1:59 ` Linus Torvalds
2010-08-18 2:14 ` David Miller
2010-08-18 4:38 ` 64-bit ppc rwsem (was: Re: [GIT] Sparc) Benjamin Herrenschmidt
2010-08-18 5:03 ` Benjamin Herrenschmidt
2010-08-18 5:28 ` 64-bit ppc rwsem David Miller
2010-08-18 5:39 ` Sam Ravnborg
2010-08-18 5:48 ` David Miller
2010-08-19 5:23 ` Benjamin Herrenschmidt
2010-08-19 5:29 ` David Miller
2010-08-19 10:24 ` Benjamin Herrenschmidt
2010-08-20 5:14 ` [PATCH 1/2] powerpc: Make rwsem use "long" type Benjamin Herrenschmidt
2010-08-24 1:37 ` Timur Tabi
2010-08-20 5:14 ` [PATCH 2/2] rwsem: Move powerpc atomic-long based implementation to asm-generic Benjamin Herrenschmidt
2010-08-20 6:43 ` Sam Ravnborg
2010-08-24 1:32 ` Benjamin Herrenschmidt
2010-08-23 4:39 ` David Miller
2010-08-23 13:44 ` 64-bit ppc rwsem Arnd Bergmann
2010-08-23 22:01 ` Benjamin Herrenschmidt [this message]
2010-08-23 22:18 ` David Miller
2010-08-24 1:31 ` Benjamin Herrenschmidt
2010-08-24 12:06 ` Arnd Bergmann
2010-08-18 2:12 ` [GIT] Sparc David Miller
2010-08-18 2:50 ` Al Viro
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