Re: single copy atomicity for double load/stores on 32-bit systems

[Peter Zijlstra <peterz@infradead.org>]

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.