Btw; I'll cc'd this to lad, as this is a topic that has been discussed there _many_ times. More specifically the SMP-sparc case has been mentioned quite a few times but without specific details. For lad-only readers, see the jackit-devel archives for the whole thread. On Wed, 13 Nov 2002, Tim Goetze wrote: [...] Actually I've spent some time studying this issue and my conclusion is that the level of atomicity provided by linux asm-arch/atomic.h is not needed to implement single-reader-single-writer lock-free buffers. More specifically, asm/atomic.h provides guaranteed ordering of reads and writes. This is not really needed in handling the l-f buffer indices. In the worst case reported write-space is larger, or read-space smaller, than the actual situation. The only problematic scenarios are that a) read returns an invalid value, and b) write stores an invalid value. These could possibly happen if reads/writes were not atomic. But so far I haven't found any platforms (with publically available specs) which don't guarantee this level of atomicity (even in the case of complex processor cache arrangements). The two architectures for which asm-xxx/atomic.h contains special code even for plain reads and writes are SMP-sparc and s390. I haven't studied s390, but at least in the case of sparcs (ref: sparc v8 arch manual), reads and writes are atomic even in a SMP configuration, although ordering is not guaranteed. More specifically, if TSO (Total Store Ordering) memory mode is selected, loads do not necessarily appear in order. If using PSO (Partial SO), stores as seen by the issuing processor can be executed out-of-order. But these restrictions aren't really a problem for l-f buffers. If I've interpreted the situation incorrectly, or someone has better information, I'd be very interested to know! Even better, example code that demonstrates atomicity problems... I have currenly access to both IA32 and Sparc-v8 multi-processor machines. -- http://www.eca.cx Audio software for Linux!