ThreadSanitizer: 運作原理分析與原始碼導讀 - 王琮瑋

請從這裡開始

What's data race?

Sync across threads

跨 Threads 執行時，誰先誰後？

Memory Model

• Define which operation will create happens before realtion across threads

透過不同語言的 memory model 可以看出先後順序的關係

• https://golang.org/ref/mem

How threadSanitizer works?

• Compile time - Compiler instrumentation
  • Memory access (r/w)
  • Sync operations (acquire/release)
  • Functions (entry/exit)
• Runtime - detecion
  • Monitor all across thread memory events
  • For each 8-byte memory address , record old access events
  • Compare new access events with all old events to detect race
• Runtime - report
  • maintain perthread
  • ge

##compile instrumentation
void foo(int *p)
`{
*p=42;
}
void foo(int *p)
{
 _tsan_fun_entry
}

Shadow State - one for every 8-byte memory

• Composed by N 64-bit shadow words
• Shadow word
  • ~ 13 bits : Thread ID
  • ~42 bits : Scalar Clock (Maintained by each Thread seperately )
  • 1 bit : IsWrite
  • 2 bits Access size (1/2/4/8)
  • 3 bits

How to detect

• Compare new events with OLD events
• New or OLD must be write
• New and OLD must overlap
• New and OLD are not synced
  • New not happens before old
  • OLD not happens before new

Compare based on local clock

• How to compare

  • OLD( Thread B , clock 1000)
  • NEW( Thread A , clock 2000)

• For each thread , maintain a clock vector for all other threads

• For each sync variable , maintain…

code related to clock

• tsan_clock.cpp#L23
• ThreadClock (clock vector for each Thread )
• SyncClock (clock vector for eacch Sync Variable)

void ThreadClock::acquire(const SyncClock *src)
{ for(inti=0;i<kMaxThreads;i++)
    clock[1]=max(clock[i],src->clock[1]);}
void ThreadClock::()

Example 1

• (T1 , Write , 0) -> (T2 , Read , 0)

Example 2

• (T1 , LOck[M1] , 0) -> (T1 , Write , 1) -> (T1 , Unlock [M1] , 2 ) -> (T2 , LOCK[M1] , 0 ) …

Why 42 bits local clock ?

• Why not increment over sync operation?
• What kinds of report is useful
• How to recover stack trace
  • Maintain a cyclic event list
  • Find the position of OLD operation

Q & A

tags: COSCUP2019 系統軟體社群議程 IB201