PACtree implementation

[saltway]

hi, the implementation of PACtree may incur segmentation fault in my experiments. I follow the tips provided here https://github.com/cosmoss-jigu/pactree/issues/1 but segmentation fault still exists I hope you can help me. Thanks

[jr-shen]

There seems to be another two bugs in the implementation of PACtree.

1. OpLogs may overlap with each other when key length is not 8, because sizeof(OpStruct) is hard-coded to 64 in PMem::getOpLog. However, when key length is not 8, sizeof(OpStruct) is not 64. I changed 64 to sizeof(OpStruct) and it seems to work.

   static void* getOpLog(int i){
   unsigned long vaddr = (unsigned long)logVaddr[0];
   //printf("vaddr :%p %p\n",vaddr, (void *)(vaddr+(64*i)));
   
       // return (void*)(vaddr + (64*i));
   return (void*)(vaddr + (sizeof(OpStruct)*i));
   }

2. Current pptr implementation may cause racing condition. When rawPtr is changed by other threads between [1], [2] or [3], [4], inconsistent vaddr may be generated. This may occur when using low compiler optimization level (like -O0) for debugging purposes because the compiler will generate two load instructions for rawPtr in each function.

   
   T *operator->() {
   int poolId = (rawPtr&MASK_POOL) >> 48; // [1]
   void *baseAddr = PMem::getBaseOf(poolId); 
   unsigned long offset = rawPtr & MASK; // [2]
   return (T *)((unsigned long)baseAddr + offset);
   }

T *getVaddr() { unsigned long offset = rawPtr & MASK; // [3] if(offset == 0){ return nullptr; }

int poolId = (rawPtr&MASK_POOL) >> 48; // [4] void *baseAddr = PMem::getBaseOf(poolId);

return (T *)((unsigned long)baseAddr + offset); }

I modified it as following:
```C
T *operator->() {
  unsigned long p = rawPtr;
  int poolId = (p&MASK_POOL) >> 48; // [1]
  void *baseAddr = PMem::getBaseOf(poolId); 
  unsigned long offset = p & MASK; // [2]
  return (T *)((unsigned long)baseAddr + offset);
}

T *getVaddr() {
  unsigned long p = rawPtr;
  unsigned long offset = p & MASK; // [3]
  if(offset == 0){
    return nullptr;
  }

  int poolId = (p&MASK_POOL) >> 48; // [4]
  void *baseAddr = PMem::getBaseOf(poolId); 

  return (T *)((unsigned long)baseAddr + offset);
}

[georgeh0214]

The segmentation fault issue does occur with inserts and occurs more frequently with the number of concurrent threads. After applying the fixes in https://github.com/cosmoss-jigu/pactree/issues/1#issuecomment-1045699089 and the one above, it still crashes occasionally. I would suggest adding the above fix to the issue in pactree's repo.