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Memory ordering matters

dotnet/runtime
Based on 3 comments
C++

When working with shared data in multithreaded environments, memory ordering is critical to prevent race conditions and ensure thread safety: 1. **Avoid transient incorrect states** - Don't write an incorrect value before replacing it with the correct one, as other threads may see the intermediate state:

Concurrency C++

Reviewer Prompt

When working with shared data in multithreaded environments, memory ordering is critical to prevent race conditions and ensure thread safety:

  1. Avoid transient incorrect states - Don’t write an incorrect value before replacing it with the correct one, as other threads may see the intermediate state:
// INCORRECT: Creates a race window where incorrect value is visible
*ppvRetAddrLocation = (void*)pfnHijackFunction;
#if defined(TARGET_ARM64)
*ppvRetAddrLocation = PacSignPtr(*ppvRetAddrLocation);
#endif

// CORRECT: Prepare final value before publishing
void* pvHijackAddr = (void*)pfnHijackFunction;
#if defined(TARGET_ARM64)
pvHijackAddr = PacSignPtr(pvHijackAddr);
#endif
*ppvRetAddrLocation = pvHijackAddr;
  1. Use atomic operations correctly - Pay careful attention to argument order in functions like InterlockedCompareExchange(destination, exchange, comparand):
// INCORRECT: Arguments in wrong order
if (versionStart != InterlockedCompareExchange(&s_stackWalkNativeToILCacheVersion, versionStart, versionStart | 1))

// CORRECT: Proper argument order
if (versionStart != InterlockedCompareExchange(&s_stackWalkNativeToILCacheVersion, versionStart | 1, versionStart))
  1. Use memory barriers when publishing shared data - Ensure supporting data is visible before publishing pointers:
// INCORRECT: May be reordered
s_stackWalkCacheSize = cacheSize;
s_stackWalkCache = newCache; // Other threads might see the pointer before size is set

// CORRECT: Memory barrier ensures proper ordering
VolatileStore(&s_stackWalkCacheSize, cacheSize);
s_stackWalkCache = newCache;

Proper memory ordering is essential for preventing subtle multithreading bugs that may only appear under high load or on specific hardware architectures.

3
Comments Analyzed
C++
Primary Language
Concurrency
Category

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