Re: [PING] [PATCH] Fix parameters of __tsan_vptr_update

[Dmitry Vyukov <dvyukov at google dot com>]

On Tue, Jan 20, 2015 at 10:09 AM, Bernd Edlinger
<bernd.edlinger@hotmail.de> wrote:
>
> Hi,
>
> On Mon, 19 Jan 2015 18:49:21, Konstantin Serebryany wrote:
>>
>> [text-only]
>>
>> On Mon, Jan 19, 2015 at 7:42 AM, Mike Stump <mikestump@comcast.net> wrote:
>>> On Jan 19, 2015, at 12:43 AM, Dmitry Vyukov <dvyukov@google.com> wrote:
>>>> I can't really make my mind on this. I would mildly prefer sleep's (if
>>>> they work reliably!).
>>>
>>> Let me state it more forcefully.
>> You don't have to convince us here.
>> I'd love to get rid of sleep calls in the tsan test suite -- they are
>> a minor but a constant annoyance.
>> But I also want to keep the tests *very simple*, i.e.
>> 1. Single file w/o any non-system includes, no linking of extra
>> libraries/objects
>> 2. Not too much extra code. (ideally, 1 line for init, 1 line for
>> "signal", 1 line for "wait")
>> 3. Strictly posix or c++11 (unless we are testing something specific)
>>
>> Your idea with barrier_wait/dlsym sounds interesting, but I can't see
>> the code in this mail thread.
>> What do I miss?
>>
>
> We discussed two alternatives to sleep:
>
> 1. step function, optionally with sched_yield to make it somewhat less busy waiting:
> __attribute__((no_sanitize_thread))
> void step (int i)
> {
>    while (__atomic_load_n (&serial, __ATOMIC_ACQUIRE) != i - 1)
>      sched_yield();
>    __atomic_store_n (&serial, i, __ATOMIC_RELEASE);
> }
> 2. tsan-invisible barriers:
>
> cat tsan_barrier.h
> /* TSAN-invisible barriers.  Link with -ldl.  */
> #include <pthread.h>
> #include <dlfcn.h>
>
> static __typeof(pthread_barrier_wait) *barrier_wait;
>
> static
> void barrier_init (pthread_barrier_t *barrier, unsigned count)
> {
>   void *h = dlopen ("libpthread.so.0", RTLD_LAZY);
>   barrier_wait = (__typeof (pthread_barrier_wait) *)
>           dlsym (h, "pthread_barrier_wait");
>   pthread_barrier_init (barrier, NULL, count);
> }
>
>
> We preferred the second alternative, because it does not do busy waiting.
> We include this header file in every positive test case and link with -ldl.


The step approach looks better to me at first sight.

Busy waiting looks like a weak argument in this context. It's
absolutely non performance-critical and a yield or usleep(10) will
solve it more than sufficiently.

I will check how complex to make its implementation invisible to tsan
(I suspect that clang does not ignore atomic operations when
no_sanitize_thread attribute is given) and whether it actually makes
more complex tests simpler to write (as compared to the barrier
approach).