Hi Peter, On Fri, Jan 27, 2023 at 12:18:13PM +0100, Peter Zijlstra wrote: > On Thu, Jan 26, 2023 at 06:33:54PM +0100, Jules Maselbas wrote: > > > @@ -58,9 +61,11 @@ static inline int generic_atomic_fetch_##op(int i, atomic_t *v) \ > > static inline void generic_atomic_##op(int i, atomic_t *v) \ > > { \ > > unsigned long flags; \ > > + int c; \ > > \ > > raw_local_irq_save(flags); \ > > - v->counter = v->counter c_op i; \ > > + c = arch_atomic_read(v); \ > > + arch_atomic_set(v, c c_op i); \ > > raw_local_irq_restore(flags); \ > > } > > This and the others like it are a bit sad, it explicitly dis-allows the > compiler from using memops and forces a load-store. Good point, I don't know much about atomic memops but this is indeed a bit sad to prevent such instructions to be used. > The alternative is writing it like: > > *(volatile int *)&v->counter c_op i; I wonder if it could be possible to write something like: *(volatile int *)&v->counter += i; 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. Thanks, -- Jules