linux-kernel - Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it

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Date:	Wed, 6 Dec 2006 11:34:52 -0800 (PST)
From:	Linus Torvalds <torvalds@...l.org>
To:	Matthew Wilcox <matthew@....cx>
cc:	Christoph Lameter <clameter@....com>,
	Russell King <rmk+lkml@....linux.org.uk>,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...l.org>,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch
 doesn't support it



On Wed, 6 Dec 2006, Matthew Wilcox wrote:
> 
> Given parisc's paucity of atomic operations (load-and-zero-32bit and
> load-and-zero-64bit), cmpxchg() is impossible to implement safely.
> There has to be something we can hook to exclude another processor
> modifying the variable.  I'm OK with using atomic_cmpxchg(); we have
> atomic_set locked against it.

How do you to the atomic bitops?

Also, I don't see quite why you think "cmpxchg()" and "atomic_cmpxchg()" 
would be different. ANY cmpxchg() needs to be atomic - if it's not, 
there's no point to the operation at all, since you'd just write it as

	if (*p == x)
		*p = y;

instead, and not use cmpxchg(). 

So yes, architectures without native support (where "native" includes 
load-locked + store-conditional) always need to

 - on UP, just disable interrupts
 - on SMP, use a spinlock (with interrupts disabled), and share that 
   spinlock with all the other atomic ops (bitops at a minimum - the 
   "atomic_t" operations have traditionally been in another "locking 
   space" because of sparc32 historic braindamage, but I'd suggest 
   sharing the same spinlock between them all).

And yeah, it sucks. You _can_ (if you really want to) make the spinlock be 
hashed based on the address of the atomic data structure. That at least 
allows you to do _multiple_ spinlocks, but let's face it, your real 
problem is _likely_ going to be cacheline bouncing, not contention, and 
then using a hashed lock won't be likely to buy you all that much.

		Linus
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