Messages in this thread | | | Date | Fri, 16 Sep 2005 23:36:28 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 2/5] atomic: introduce atomic_inc_not_zero |
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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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