[tsan] Is this false warning when using std::condition_variable like this?

[Rereflyer]

When compiling the following code with gcc7, utilizing thread sanitizer

#include <pthread.h>
#include <stdio.h>

int Global = 0x12345678;
pthread_cond_t condition = PTHREAD_COND_INITIALIZER;
pthread_mutex_t mu = PTHREAD_MUTEX_INITIALIZER;

void *Thread1(void *x) {
    pthread_mutex_lock(&mu);
    pthread_cond_wait(&condition, &mu);
    printf("print Global:0x%X\n",Global);
    return x;
}

void *Thread2(void *x) {
    char* c1 = (char*)(&Global) + 3;
    (*c1) = 1;
    pthread_cond_signal(&condition);
    return x;
}

int main() {
    pthread_t t1;
    pthread_t t2;
    pthread_create(&t1, NULL, Thread1, NULL);
    pthread_create(&t2, NULL, Thread2, NULL);
    pthread_join(t1, NULL);
    pthread_join(t2, NULL);
    return Global;
}

Generated warnings when running the program:

==================
WARNING: ThreadSanitizer: data race (pid=126626)
  Read of size 4 at 0x55de3589e010 by thread T1 (mutexes: write M0):
    #0 Thread1 /home/xx/work/verify/c++/tsan/sim_oneDNN_wait.c:11 (sim_oneDNN_wait.out+0x125d)
    #1 <null> <null> (libtsan.so.0+0x296ad)

  Previous write of size 1 at 0x55de3589e013 by thread T2:
    #0 Thread2 /home/xx/work/verify/c++/tsan/sim_oneDNN_wait.c:17 (sim_oneDNN_wait.out+0x12b9)
    #1 <null> <null> (libtsan.so.0+0x296ad)

  Location is global 'Global' of size 4 at 0x55de3589e010 (sim_oneDNN_wait.out+0x000000004010)

  Mutex M0 (0x55de3589e080) created at:
    #0 pthread_mutex_lock <null> (libtsan.so.0+0x3fadb)
    #1 Thread1 /home/xx/work/verify/c++/tsan/sim_oneDNN_wait.c:9 (sim_oneDNN_wait.out+0x123e)
    #2 <null> <null> (libtsan.so.0+0x296ad)

  Thread T1 (tid=126628, running) created by main thread at:
    #0 pthread_create <null> (libtsan.so.0+0x2bcee)
    #1 main /home/xx/work/verify/c++/tsan/sim_oneDNN_wait.c:25 (sim_oneDNN_wait.out+0x1320)

  Thread T2 (tid=126629, finished) created by main thread at:
    #0 pthread_create <null> (libtsan.so.0+0x2bcee)
    #1 main /home/xx/work/verify/c++/tsan/sim_oneDNN_wait.c:26 (sim_oneDNN_wait.out+0x133d)

SUMMARY: ThreadSanitizer: data race /home/xx/work/verify/c++/tsan/sim_oneDNN_wait.c:11 in Thread1
==================
print Global:0x1345678
ThreadSanitizer: reported 1 warnings

I am building the code with the following command:
gcc sim_wait.c -o sim_wait.out -fsanitize=thread -g

In my opinion, Thread1 will wait condition variale so write global variable must happen before read global variable, is this false warning when tsan report data race? Thanks.

[tavianator]

pthread_cond_wait() is allowed to return spuriously:

When using condition variables there is always a Boolean predicate involving shared variables associated with each condition wait that is true if the thread should proceed. Spurious wakeups from the pthread_cond_timedwait() or pthread_cond_wait() functions may occur. Since the return from pthread_cond_timedwait() or pthread_cond_wait() does not imply anything about the value of this predicate, the predicate should be re-evaluated upon such return.

https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_wait.html