On 18/05/2016 17:05, Sergey Fedorov wrote:
> Please look at this:
>
> $ cat >a.c <<EOF
> int atomic_exchange(int *x, int v)
> {
> return __atomic_exchange_n(x, v, __ATOMIC_ACQUIRE);
> }
>
> int sync_lock_test_and_set(int *x, int v)
> {
> __sync_lock_test_and_set(x, v);
> }
> EOF
>
> Disassembly of section .text:
>
> 00000000 <atomic_exchange>:
> 0: e1902f9f ldrex r2, [r0]
> 4: e1803f91 strex r3, r1, [r0]
> 8: e3530000 cmp r3, #0
> c: 1afffffb bne 0 <atomic_exchange>
> 10: ee070fba mcr 15, 0, r0, cr7, cr10, {5}
> 14: e1a00002 mov r0, r2
> 18: e12fff1e bx lr
>
> 00000078 <sync_lock_test_and_set>:
> 78: e1902f9f ldrex r2, [r0]
> 7c: e1803f91 strex r3, r1, [r0]
> 80: e3530000 cmp r3, #0
> 84: 1afffffb bne 78 <sync_lock_test_and_set>
> 88: ee070fba mcr 15, 0, r0, cr7, cr10, {5}
> 8c: e12fff1e bx lr
>
>
> atomic_compare_exchange() looks pretty good, doesn't it? Could we use it
> to implement qemu_spin_lock()?
I guess you mean atomic_exchange? That one looks good, indeed it's
equivalent to __sync_lock_test_and_set.
But honestly I think it would be even better to just use
__sync_lock_test_and_set in the spinlock implementation and not add this
to atomics.h. There's already enough issues with the current subset of
atomics, I am not really happy to add non-SC read-modify-write
operations to the mix.
Thanks,
Paolo
