Re: [PATCH] ARC: Explicitly set ARCH_SLAB_MINALIGN = 8

[Vineet Gupta <vineet.gupta1@synopsys.com>]

+CC some folks interested in alignment stuff in the past.

On 2/12/19 9:30 AM, David Laight wrote:
> From:  Vineet Gupta
>> Sent: 12 February 2019 17:17
>>
>> On 2/8/19 2:55 AM, Alexey Brodkin wrote:
>>> By default ARCH_SLAB_MINALIGN is defined in "include/linux/slab.h" as
>>> "__alignof__(unsigned long long)" which looks fine but not for ARC.
>>
>> Just for the record, the issue happens because a LLOCKD (exclusive 64-bit load)
>> was trying to use a 32-bit aligned effective address (for atomic64_t), not allowed
>> by ISA (LLOCKD can only take 64-bit aligned address, even when the CPU has
>> unaligned access enabled).
>>
>> This in turn was happening because this word is embedded in some other struct and
>> happens to be 4 byte aligned
>>
>>
>>> ARC tools ABI sets align of "long long" the same as for "long" = 4
>>> instead of 8 one may think of.
> 
> Right, but __alignof__() doesn't have to return the alignment that would
> be used for a data item of the specified type.
> (Read the gcc 'bug' info for gory details.)
> 
> On i386 __alignof__(long long) is 8, but structure members of type 'long long'
> are 4 byte aligned and the alignment of a structure with a 'long long' member
> is only 4.
> (Although the microsoft compiler returns 4.)

Exactly my point that this fudging of outer alignment is no magic bullet.

> 
>> Right, this was indeed unexpected and not like most other arches. ARCv2 ISA allows
>> regular 64-bit loads/stores (LDD/STD) to take 32-bit aligned addresses. Thus ABI
>> relaxing the alignment for 64-bit data potentially causes more packing and less
>> space waste. But on the flip side we need to waste space at arbitrary places liek
>> this.
>>
>> So this is all good and theory, but I'm not 100% sure how slab alignment helps
>> here (and is future proof). So the outer struct with embedded atomic64_t was
>> allocated via slab and your patch ensures that outer struct is 64-bit aligned ?
> 
> Presumable 'atomic64_t' has an alignment attribute to force 8 byte alignment.

It does for ARC

typedef struct {
        aligned_u64 counter;
} atomic64_t;

But what was your point ?

> 
>> But how does that guarantee that all embedded atomic64_t in there will be 64-bit
>> aligned (in future say) in the light of ARC ABI and the gcc bug/feature which
>> Peter alluded to
>>
>>    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54188
>>    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10360
>>
>>> Thus slab allocator may easily allocate a buffer which is 32-bit aligned.
>>> And most of the time it's OK until we start dealing with 64-bit atomics
>>> with special LLOCKD/SCONDD instructions which (as opposed to their 32-bit
>>> counterparts LLOCK/SCOND) operate with full 64-bit words but those words
>>> must be 64-bit aligned.
>>
>> Some of this text needed to go above to give more context.
> 
> I suspect the slab allocator should be returning 8 byte aligned addresses
> on all systems.... 

why ? As I understand it is still not fool proof against the expected alignment of
inner members. There ought to be a better way to enforce all this.