On Wed, Jan 27, 2016 at 06:43:36PM +0000, Mathieu Desnoyers wrote: > ----- On Jan 27, 2016, at 1:03 PM, Josh Triplett josh@xxxxxxxxxxxxxxxx wrote: > > > On Wed, Jan 27, 2016 at 05:36:48PM +0000, Mathieu Desnoyers wrote: > >> ----- On Jan 27, 2016, at 12:24 PM, Thomas Gleixner tglx@xxxxxxxxxxxxx wrote: > >> > On Wed, 27 Jan 2016, Josh Triplett wrote: > >> >> With the dynamic allocation removed, this seems sensible to me. One > >> >> minor nit: s/int32_t/uint32_t/g, since a location intended to hold a CPU > >> >> number should never need to hold a negative number. > >> > > >> > You try to block the future of computing: https://lwn.net/Articles/638673/ > >> > >> Besides impossible architectures, there is actually a use-case for > >> signedness here. It makes it possible to initialize the cpu number > >> cache to a negative value, e.g. -1, in userspace. Then, a check for > >> value < 0 can be used to figure out cases where the getcpu_cache > >> system call is not implemented, and where a fallback (vdso or getcpu > >> syscall) needs to be used. > >> > >> This is why I have chosen a signed type for the cpu cache so far. > > > > If getcpu_cache doesn't exist, you'll get ENOSYS. If getcpu_cache > > returns 0, then you can assume the kernel will give you a valid CPU > > number. > > I'm referring to the code path that read the content of the cache. > This code don't call the getcpu_cache system call each time (this > would defeat the entire purpose of this cache), but still has to > know whether it can rely on the cache content to contain the current > CPU number. Seeing a "-1" there is a nice way to tell the fast path > that it needs to go through a fallback. > > Or perhaps you have another mechanism in mind for that ? How do > you intend to communicate the ENOSYS from the kernel to all > eventual readers of the cache, without adding extra function > call overhead on the fast path ? Have the fast path assume the cache, without even checking for -1; only use that fast path if getcpu_cache exists. If you don't have getcpu_cache, don't even attempt to use the fast path; substitute in a fallback implementation. Don't have a conditional in either version; just decide which version to use based on system capabilities. Alternatively, use the implementation you have with a placeholder value, and just use 0xFFFFFFFF as the placeholder; that seems no more or less valid. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
