Re: [LAD] [LAU] Simple, easy multithreaded circular buffer library for Linux?

[Switched from LAU to LAD] Forget I said anything about context switches. Those don't matter. The concern here is SMP systems. The bug in the original code is += operator used to increment the read and write pointers. That operator generates the addl instruction, which is not atomic. It needs to be locked on SMP. The reason Olivier's patch works is because the increment is done on a temp and then assinged (using plain old =) to the shared variable. The assignment *is* atomic (since size_t is 4 bytes and presumably aligned to a 4-byte boundary). I can't prove this right now, but I think I'm correct. That means it may be possible to make the original code SMP-safe without the (subtle) change in semantics Olivier's patch makes. Like increment a temp, store it, mask the temp, store it again. You can't write lock-free data structures without atomic operations.

While the x86 doesn't need any memory barriers (all stores are seen by all other CPUs), other platforms could (someone mentioned PowerPC). The code still needs to be patched for that; just make the x86 version no-ops.