Slim Reader/Writer locks deadlocking - c++

Question

For some reason a simple Slim Reader/Writer lock pattern is deadlocking and I don't know why. I have two threads who share the lock (one using exclusive ownership and one using shared ownership).
For some reason, when a second exclusive locking thread is introduced it gets extremely slow for no discernable reason.
my code:
#include <iostream>
#include <Windows.h>
SRWLOCK mainLock;
char myglobalvar;
int main()
{
myglobalvar = '0';
HANDLE t1 = CreateThread(nullptr, 0, [](void*)->DWORD
{
while (true)
{
AcquireSRWLockShared(&mainLock);
myglobalvar = 'S';
Sleep(500);
ReleaseSRWLockShared(&mainLock);
}
return 0;
}, nullptr, 0, nullptr);
Sleep(50);
HANDLE t2 = CreateThread(nullptr, 0, [](void*)->DWORD
{
while (true)
{
AcquireSRWLockExclusive(&mainLock);
myglobalvar = 'E';
ReleaseSRWLockExclusive(&mainLock);
}
return 0;
}, nullptr, 0, nullptr);
for (int i = 0; i < 100; i++)
{
AcquireSRWLockExclusive(&mainLock);
std::cout << myglobalvar << '\n';
ReleaseSRWLockExclusive(&mainLock);
Sleep(50);
}
}

Answer 1

Slim Reader/Writer (SRW) Locks
Shared mode, which grants shared read-only access to multiple reader
threads, which enables them to read data from the shared resource
concurrently.
As #Peter said, there is no chance for main thread to acquire the lock because of t2. And your slow feeling is the only std::cout.
The following code works fine for me.
#include <iostream>
#include <Windows.h>
SRWLOCK mainLock;
char myglobalvar;
int main()
{
myglobalvar = '0';
HANDLE t1 = CreateThread(nullptr, 0, [](void*)->DWORD
{
while (true)
{
AcquireSRWLockShared(&mainLock);
//myglobalvar = 'S';
std::cout << myglobalvar << '\n';
ReleaseSRWLockShared(&mainLock);
Sleep(250);
}
return 0;
}, nullptr, 0, nullptr);
Sleep(50);
HANDLE t2 = CreateThread(nullptr, 0, [](void*)->DWORD
{
while (true)
{
AcquireSRWLockExclusive(&mainLock);
myglobalvar = 'E';
ReleaseSRWLockExclusive(&mainLock);
Sleep(500);
}
return 0;
}, nullptr, 0, nullptr);
for (int i = 0; i < 100; i++)
{
AcquireSRWLockExclusive(&mainLock);
myglobalvar = 'S';
//std::cout << myglobalvar << '\n';
ReleaseSRWLockExclusive(&mainLock);
Sleep(500);
}
}