[Valgrind-users] Helgrind data race question

[Phil L. <plo...@sa...>]

According to the helgrind manual "When a mutex is unlocked by thread T1 and later (or immediately) locked by thread T2, then the memory accesses in T1 prior to the unlock must happen-before those in T2 after it acquires the lock".

Two possible ways to interpret this:

Scenario #1:
int shared_value;
pthread_mutex_t lock;

Thread 1:

pthread_mutex_lock(&lock);
shared_value = 10;
pthread_mutex_unlock(&lock);

Thread 2:

pthread_mutex_lock(&lock);
... = shared_value;
pthread_mutex_unlock(&lock);

For this, no data race on shared_value should be reported;

Scenario #2:

pthread_mutex_t lock;
int* shared_ptr;

Thread 1:

int* my_ptr = new int;
*my_ptr = 10;
pthread_mutex_lock(&lock);
shared_ptr = my_ptr;
pthread_mutex_unlock(&lock);

Thread 2:
pthread_mutex_lock(&lock);
int* my_ptr = shared_ptr;
pthread_mutex_unlock(&lock);
... = *my_ptr;

In this case, there should be no data race on shared_ptr, similar to scenario #1.  Will helgrind report a data race on the memory shared_ptr points to, or is memory only deemed to be safe while a semaphore is locked.

Phil
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Phil Longstaff
Senior Software Engineer
x2904