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

[Peter Zijlstra <peterz@infradead.org>]

On Thu, May 30, 2019 at 07:16:36PM +0000, Vineet Gupta wrote:
> On 5/30/19 11:55 AM, Paul E. McKenney wrote:
> >
> >> 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 ....)
> >>
> >> I presume (and the question) that lkmm doesn't expect such 8 byte load/stores to
> >> be atomic unless 8-byte aligned
> > I would not expect 8-byte accesses to be atomic on 32-bit systems unless
> > some special instruction was in use.  But that usually means special
> > intrinsics or assembly code.
> 
> Thx for confirming.
> 
> In cases where we *do* expect the atomicity, it seems there's some existing type
> checking but isn't water tight.
> e.g.
> 
> #define __smp_load_acquire(p)                        \
> ({                                    \
>     typeof(*p) ___p1 = READ_ONCE(*p);                \
>     compiletime_assert_atomic_type(*p);                \
>     __smp_mb();                            \
>     ___p1;                                \
> })
> 
> #define compiletime_assert_atomic_type(t)                \
>     compiletime_assert(__native_word(t),                \
>         "Need native word sized stores/loads for atomicity.")
> 
> #define __native_word(t) \
>     (sizeof(t) == sizeof(char) || sizeof(t) == sizeof(short) || \
>      sizeof(t) == sizeof(int) || sizeof(t) == sizeof(long))
> 
> 
> So it won't catch the usage of 4 byte aligned long long which gcc targets to
> single double load instruction.

Yes, we didn't do those because that would result in runtime overhead.

We assume natural alignment for any type the hardware can do.