Re: [PATCH 2/5] atomic: introduce atomic_inc_not_zero

[Roman Zippel <zippel@linux-m68k.org>]

Hi,

On Thu, 15 Sep 2005, Nick Piggin wrote:

> Roman: any ideas about what you would prefer? You'll notice
> atomic_inc_not_zero replaces rcuref_inc_lf, which is used several times
> in the VFS.

In the larger picture I'm not completely happy with these scalibilty 
patches, as they add extra overhead at the lower end. On a UP system in 
general nothing beats:

	spin_lock();
	if (*ptr)
		ptr += 1;
	spin_unlock();

The main problem is here that the atomic functions are used in two basic 
situation:

1) interrupt synchronization
2) multiprocessor synchronization

The atomic functions have to assume both, but on UP systems it often is 
a lot cheaper if they don't have to synchronize with interrupts. So 
replacing a spinlock with a few atomic operations can hurt UP performance.

bye, Roman
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