[LITMUS^RT] New release: 2012.3

[Felipe Cerqueira]

2013/1/16 Björn Brandenburg <bbb at mpi-sws.org>

>
> On Jan 16, 2013, at 5:31 PM, Glenn Elliott <gelliott at cs.unc.edu> wrote:
>
> >  Also, and probably the biggest issue going forward, testing on ARM is
> difficult since ARM kernels are so fragmented.  We have to port Litmus to
> each unique kernel devised for each of our ARM platforms.
>
> One more reason to rebase to a newer Linux version; they consolidated the
> ARM support in recent versions.
>

Unfortunately 64-bit processors will only start to be produced in 2014
(Cortex A50 series).

Good news though...
I found that Linux implements atomic64_cmpxchg() in
arch/arm/include/asm/atomic.h, even in the 3.0 kernel. Probably for the
older ARMv6k ARM boards.
In the first v7 instruction set ARM removed support for LDREXD, and it only
got back in v7-AR. The problem is that this is not treated in Linux headers.

3.7 kernel does a little better job by providing access to
atomic64_cmpxchg() via another macro:

#define cmpxchg64(ptr, o, n)                        \
    ((__typeof__(*(ptr)))atomic64_cmpxchg(container_of((ptr),    \
                        atomic64_t,        \
                        counter),        \
                          (unsigned long)(o),    \
                          (unsigned long)(n)))


/*
 * ARMv6 UP and SMP safe atomic ops.  We use load exclusive and
 * store exclusive to ensure that these are atomic.  We may loop
 * to ensure that the update happens.
 */
[...]

static inline u64 atomic64_cmpxchg(atomic64_t *ptr, u64 old, u64 new)
{
    u64 oldval;
    unsigned long res;

    smp_mb();

    do {
        __asm__ __volatile__("@ atomic64_cmpxchg\n"
        "ldrexd        %1, %H1, [%3]\n"
        "mov        %0, #0\n"
        "teq        %1, %4\n"
        "teqeq        %H1, %H4\n"
        "strexdeq    %0, %5, %H5, [%3]"
        : "=&r" (res), "=&r" (oldval), "+Qo" (ptr->counter)
        : "r" (&ptr->counter), "r" (old), "r" (new)
        : "cc");
    } while (res);

    smp_mb();

    return oldval;
}


So, we can simply edit the normal cmpxchg() macro, put a new ifdef
CONFIG_ARMV7AR and add support to 64-bit.
That way, we won`t need to modify Litmus code.

Thanks,
Felipe
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