Hi Huacai & Arnd, Please have a look at riscv qspinlock V5: https://lore.kernel.org/linux-riscv/a498a2ff-2503-b25c-53c9-55f5f2480bf6@xxxxxxxxxxxxx/T/#t On Mon, Jun 20, 2022 at 5:50 PM Huacai Chen <chenhuacai@xxxxxxxxxx> wrote: > > Hi, > > On Mon, Jun 20, 2022 at 12:11 AM Arnd Bergmann <arnd@xxxxxxxx> wrote: > > > > On Sun, Jun 19, 2022 at 5:48 PM Guo Ren <guoren@xxxxxxxxxx> wrote: > > > > > > On Sat, Jun 18, 2022 at 1:40 PM Arnd Bergmann <arnd@xxxxxxxx> wrote: > > > > > > > > On Sat, Jun 18, 2022 at 1:19 AM Guo Ren <guoren@xxxxxxxxxx> wrote: > > > > > > > > > > > static inline u32 arch_xchg32(u32 *ptr, u32 x) {...} > > > > > > static inline u64 arch_xchg64(u64 *ptr, u64 x) {...} > > > > > > > > > > > > #ifdef CONFIG_64BIT > > > > > > #define xchg(ptr, x) (sizeof(*ptr) == 8) ? \ > > > > > > arch_xchg64((u64*)ptr, (uintptr_t)x) \ > > > > > > arch_xchg32((u32*)ptr, x) > > > > > > #else > > > > > > #define xchg(ptr, x) arch_xchg32((u32*)ptr, (uintptr_t)x) > > > > > > #endif > > > > > > > > > > The above primitive implies only long & int type args are permitted, right? > > > > > > > > The idea is to allow any scalar or pointer type, but not structures or > > > > unions. If we need to deal with those as well, the macro could be extended > > > > accordingly, but I would prefer to limit it as much as possible. > > > > > > > > There is already cmpxchg64(), which is used for types that are fixed to > > > > 64 bit integers even on 32-bit architectures, but it is rarely used except > > > > to implement the atomic64_t helpers. > > > A lot of 32bit arches couldn't provide cmpxchg64 (like arm's ldrexd/strexd). > > > > Most 32-bit architectures also lack SMP support, so they can fall back to > > the generic version from include/asm-generic/cmpxchg-local.h > > > > > Another question: Do you know why arm32 didn't implement > > > HAVE_CMPXCHG_DOUBLE with ldrexd/strexd? > > > > I think it's just fairly obscure, the slub code appears to be the only > > code that would use it. > > > > > > > > > > 80% of the uses of cmpxchg() and xchg() deal with word-sized > > > > quantities like 'unsigned long', or 'void *', but the others are almost > > > > all fixed 32-bit quantities. We could change those to use cmpxchg32() > > > > directly and simplify the cmpxchg() function further to only deal > > > > with word-sized arguments, but I would not do that in the first step. > > > Don't forget cmpxchg_double for this cleanup, when do you want to > > > restart the work? > > > > I have no specific plans at the moment. If you or someone else likes > > to look into it, I can dig out my old patch though. > > > > The cmpxchg_double() call seems to already fit in, since it is an > > inline function and does not expect arbitrary argument types. > Thank all of you. :) > > As Rui and Xuerui said, ll and sc in LoongArch both have implicit full > barriers, so there is no "relaxed" version. > > The __WEAK_LLSC_MB in __cmpxchg_small() have nothing to do with ll and > sc themselves, we need a barrier at the branch target just because > Loongson-3A5000 has a hardware flaw (and will be fixed in > Loongson-3A6000). > > qspinlock just needs xchg_small(), but cmpxchg_small() is also useful > for percpu operations. So I plan to split this patch to two: the first > add xchg_small() and cmpxchg_small(), the second enable qspinlock. > > Huacai > > > > > Arnd -- Best Regards Guo Ren ML: https://lore.kernel.org/linux-csky/
