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SubjectRe: [PATCH 2/5] atomic: introduce atomic_inc_not_zero
Roman Zippel <zippel@linux-m68k.org> wrote:
>
> 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.
>

Nope. On uniprocessor systems, atomic_foo() doesn't actually do the
buslocked atomic thing.

#ifdef CONFIG_SMP
#define LOCK "lock ; "
#else
#define LOCK ""
#endif

On x86, at least. Other architectures can do the same thing if they have
an atomic-wrt-IRQs add and sub.

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