Re: [igt-dev] [PATCH i-g-t 3/4] i915/gem_create: use __atomic_* instead of __sync_*

Guillaume Tucker
2019-06-13T13:03:17Z

On 06/06/2019 08:20, Ser, Simon wrote:
> On Mon, 2019-06-03 at 12:54 +0100, Guillaume Tucker wrote:
>> Replace calls to the older __sync_* functions with the new __atomic_*
>> standard ones. This fixes builds on some architectures, in particular
>> MIPS which doesn't have __sync_add_and_fetch_8 and
>> __sync_val_compare_and_swap_8 for 64-bit variable handling.
>
> Can't we use the C11 atomics from stdatomic.h instead?
>
> For instance:
> https://en.cppreference.com/w/c/atomic/atomic_compare_exchange

Yes, thanks for pointing this out. The stdatomic.h macros also
use the __atomic_* functions so the libatomic dependency remains
the same, but it's a bit better than using the __atomic_*
functions directly. In particular, stdatomic.h enforces the use
of the _Atomic type modifier which seems very important esp for
pointers to atomic variables. That's also being fixed in the v2.

Guillaume

>> Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
>> ---
>> tests/Makefile.am | 2 +-
>> tests/i915/gem_create.c | 12 ++++++++++--
>> 2 files changed, 11 insertions(+), 3 deletions(-)
>>
>> diff --git a/tests/Makefile.am b/tests/Makefile.am
>> index 5097debf629c..18a0f1f20592 100644
>> --- a/tests/Makefile.am
>> +++ b/tests/Makefile.am
>> @@ -90,7 +90,7 @@ AM_LDFLAGS = -Wl,--as-needed
>> drm_import_export_CFLAGS = $(AM_CFLAGS) $(THREAD_CFLAGS)
>> drm_import_export_LDADD = $(LDADD) -lpthread
>> gem_create_CFLAGS = $(AM_CFLAGS) $(THREAD_CFLAGS)
>> -gem_create_LDADD = $(LDADD) -lpthread
>> +gem_create_LDADD = $(LDADD) -lpthread -latomic
>> gem_close_race_CFLAGS = $(AM_CFLAGS) $(THREAD_CFLAGS)
>> gem_close_race_LDADD = $(LDADD) -lpthread
>> gem_ctx_thrash_CFLAGS = $(AM_CFLAGS) $(THREAD_CFLAGS)
>> diff --git a/tests/i915/gem_create.c b/tests/i915/gem_create.c
>> index 43cbf45f289b..a4aeb94b3f93 100644
>> --- a/tests/i915/gem_create.c
>> +++ b/tests/i915/gem_create.c
>> @@ -156,6 +156,14 @@ static void invalid_nonaligned_size(int fd)
>> gem_close(fd, create.handle);
>> }
>>
>> +static uint64_t atomic_compare_swap_u64(uint64_t *ptr, uint64_t oldval,
>> + uint64_t newval)
>> +{
>> + __atomic_compare_exchange_n(ptr, &oldval, newval, 0,
>> + __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
>> + return oldval;
>> +}
>> +
>> static uint64_t get_npages(uint64_t *global, uint64_t npages)
>> {
>> uint64_t try, old, max;
>> @@ -165,7 +173,7 @@ static uint64_t get_npages(uint64_t *global, uint64_t npages)
>> old = max;
>> try = 1 + npages % (max / 2);
>> max -= try;
>> - } while ((max = __sync_val_compare_and_swap(global, old, max)) != old);
>> + } while ((max = atomic_compare_swap_u64(global, old, max)) != old);
>>
>> return try;
>> }
>> @@ -202,7 +210,7 @@ static void *thread_clear(void *data)
>> }
>> gem_close(i915, create.handle);
>>
>> - __sync_add_and_fetch(&arg->max, npages);
>> + __atomic_add_fetch(&arg->max, npages, __ATOMIC_SEQ_CST);
>> }
>>
>> return NULL;