On Fri, Jan 27, 2023 at 03:34:33PM +0100, Peter Zijlstra wrote: > > I also noticed that GCC has some builtin/extension to do such things, > > __atomic_OP_fetch and __atomic_fetch_OP, but I do not know if this > > can be used in the kernel. > > On a per-architecture basis only, the C/C++ memory model does not match > the Linux Kernel memory model so using the compiler to generate the > atomic ops is somewhat tricky and needs architecture audits. Hijack this thread a little bit, but while we are at it, do you think it makes sense that we have a config option that allows archs to implement LKMM atomics via C11 (volatile) atomics? I know there are gaps between two memory models, but the option is only for fallback/generic implementation so we can put extra barriers/orderings to make things guaranteed to work. It'll be a code version of this document: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0124r7.html (although I realise there may be a few mistakes in that doc since I wasn't familiar with C11 memory model when I wrote part of the doc, but these can be fixed) Another reason I ask is that since Rust is coming, we need to provide our LKMM atomics in Rust so that C code and Rust code can talk via same atomic variables, since both sides need to use the same memory model. My choices are: 1. Using FFI to call Linux atomic APIs: not inline therefore not efficient. 2. Implementing Rust LKMM atomics in asm: much more work although I'm OK if we have to do it. 3. Implementing Rust LKMM atomics with standard atomics (i.e. C/C++ atomics): * Requires Rust has "volatile" atomics, which is WIP but looks promising * Less efficient compared to choice #2 but more efficient compared to choice #1 Ideally, choice #2 is the best option for all architectures, however, if we have the generic implementation based on choice #3, for some archs it may be good enough. Thoughts? [Cc LKMM and Rust people] Regards, Boqun
