On Thu, May 30, 2019 at 11:22:42AM -0700, Vineet Gupta wrote: > Hi Peter, > > Had an interesting lunch time discussion with our hardware architects pertinent to > "minimal guarantees expected of a CPU" section of memory-barriers.txt > > > | (*) These guarantees apply only to properly aligned and sized scalar > | variables. "Properly sized" currently means variables that are > | the same size as "char", "short", "int" and "long". "Properly > | aligned" means the natural alignment, thus no constraints for > | "char", two-byte alignment for "short", four-byte alignment for > | "int", and either four-byte or eight-byte alignment for "long", > | on 32-bit and 64-bit systems, respectively. > > > I'm not sure how to interpret "natural alignment" for the case of double > load/stores on 32-bit systems where the hardware and ABI allow for 4 byte > alignment (ARCv2 LDD/STD, ARM LDRD/STRD ....) Natural alignment: !((uintptr_t)ptr % sizeof(*ptr)) For any u64 type, that would give 8 byte alignment. the problem otherwise being that your data spans two lines/pages etc.. > I presume (and the question) that lkmm doesn't expect such 8 byte load/stores to > be atomic unless 8-byte aligned > > ARMv7 arch ref manual seems to confirm this. Quoting > > | LDM, LDC, LDC2, LDRD, STM, STC, STC2, STRD, PUSH, POP, RFE, SRS, VLDM, VLDR, > | VSTM, and VSTR instructions are executed as a sequence of word-aligned word > | accesses. Each 32-bit word access is guaranteed to be single-copy atomic. A > | subsequence of two or more word accesses from the sequence might not exhibit > | single-copy atomicity > > While it seems reasonable form hardware pov to not implement such atomicity by > default it seems there's an additional burden on application writers. They could > be happily using a lockless algorithm with just a shared flag between 2 threads > w/o need for any explicit synchronization. If you're that careless with lockless code, you deserve all the pain you get. > But upgrade to a new compiler which > aggressively "packs" struct rendering long long 32-bit aligned (vs. 64-bit before) > causing the code to suddenly stop working. Is the onus on them to declare such > memory as c11 atomic or some such. When a programmer wants guarantees they already need to know wth they're doing. And I'll stand by my earlier conviction that any architecture that has a native u64 (be it a 64bit arch or a 32bit with double-width instructions) but has an ABI that allows u32 alignment on them is daft.
