LLVM Instrumenting C++ lock and unlock functions - llvm

Question

Intent: Given some source code, my intent is to instrument entry/exit of critical sections and associated lock/s.
Approach: I used a function pass which basically iterate over all direct function calls, and check name against my {mutex}.lock() and {mutex}.unlock() mangled names.
Issue: I don't know how to get mutex instance on which lock() is called and then pass it to my hook function(ex. enter_cs(mtx)) as argument.
I looked at .ll code and saw this
call void #_ZNSt3__15mutex4lockEv(%"class.std::__1::mutex"* nonnull dereferenceable(64) #mtx)
I don't know how can I get access to #mtx.
I am new to LLVM so please help. Also if there is better approach for overall problem, please explain!
sample code:
mutex mtx;
void enter_cs(mutex mtx) {
/** some code **/
return;
}
void exit_cs(mutex mtx) {
/** some code **/
return;
}
int main() {
mtx.lock();
/** LLVM inserted call: enter_cs(mtx) **/
// critical section code //
mtx.unlock();
/** LLVM inserted call: exit_cs(mtx) **/
return 0;
}
LLVM Pass:
static StringRef mutexLock = "_ZNSt3__15mutex4lockEv";
bool runOnFunction(Function &F) override {
for(BasicBlock &bb : F) {
for(Instruction &i : bb) {
if(!isa<CallInst>(i)) continue; // should be a call instruction
CallInst *ci = cast<CallInst>(&i);
Function* f = ci->getCalledFunction();
if(!f) continue; // should be a direct call
if(f->getName()==mutexLock) {
errs() << "locked" << ", From: " << F.getName() << "\n";
/** enter_cs({mutex object on which lock is called}) **/
}
}
}
return false;
}