WWDC 2016 Thread Sanitizer and static analysis

Thread Sanitizer(TSan)

Use of uninitialized mutexes
Thread leaks (missing pthread_join)
Unsafe calls in signal handlers(ex:malloc)
Unlock from wrong thread

In Xcode

1. Edit Scheme - Diagnostics tab
2. "Enable Thread Sanitizer" checkbox
3. Build and Run
4. View all of the generated runtime issues
5. Can choose to break on every issue

Data Race

Multiple threads access the same memory without synchronization
At least one access is a write
May end up with any value or even memory corruption!

Reasons for Data Races

Can indicate a problem with the structure of your program
Often means that synchronization is missing

建议使用同步队列来保证数据竞争的发生

Choosing the Right Synchronization API

1. Use GCD
   Dispatch racy accesses to the same serial aueue
2. Use pthread API,NSLock
   pthread_mutex_lock()to synchronize accesses
3. New os_unfair_lock(use instead of OSSpinLock)
4. Atomic operations

More in WWDC
Concurrent Programming With GCD in Swift 3

没有办法捕获到所有的数据竞争和其他情况，因为有上限,会根据规律或者随机替换原有的线程操作(超过四个有可能会失效？)

Detecting a Race

Every thread stores (in thread local):
Thread's own timestamp
The timestamps for other thread that establish the points of synchronization
Timestamps are incremented on every memory access

Compiler Instruments memory Accesses

For every access:
Records the information about the access
Checks if that access participates in a race

Thread Sanitizer

Timing does not matter
Can detect races even if they did not manifest during the particular run
The more run time coverage the better
Run all your tests with TSan!

Find Bugs Without Running Code

Does not require running code (unlike sanitizers)
Great at catching hard to reproduce edge-case bugs
Supported only for C,C++,and Object-C

Analysis

Clang Static Analyzer in Xcode

1. Product > Analyze or
   Product > Analyze "SingleFile.m"
2. View in Issue Navigator

3. Explore Issue Path

   
   
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