Re: [boost] Help needed for shared_ptr issue on iPad2 (dual core ARM)

Subject: Re: [boost] Help needed for shared_ptr issue on iPad2 (dual core ARM)
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2011-03-28 18:31:01

On 28/03/2011 14:03, Peter Dimov wrote:
> Hi everyone,
>
> Ticket #5372:
>
> https://svn.boost.org/trac/boost/ticket/5372
>
> says that shared_ptr's ARM spinlock implementation (which uses the swp
> instruction) doesn't work properly on iPad2 (which has a dual core ARM
> processor). The sample program in the ticket compares it to a loop using
> __sync_fetch_and_add, which means that the __sync intrinsics are
> implemented by the compiler the submitter is using. These didn't work on
> gcc for ARM when we tested them, but may have been added meanwhile. (I
> can see some code samples that test for 4.4, but the official docs state
> that ARM intrinsics are only supported on Linux before 4.6, which was
> released yesterday.)
>
> So, we have two questions; first, why does the swp-based spinlock fail,
> and second, how can we detect support for __sync intrinsics and use them.
>
> Anybody with ARM knowledge and iPad2 development access?

It appears SWP does not work across multiple cores because it doesn't 
perform a memory barrier.

Wrap the code in calls to DMB or better yet, rewrite it to use LDREX/STREX.

I'll test on my dual-core Cortex-A9 when I have the time.