x86 Atomicity

lock (prefix)

• Makes the memory instruction atomic
• Locks memory bus for duration of instruction
  • Expensive!
• All lock instructions totally ordered
• Other memory instructions cannot be re-ordered with locked ones

xchg

• Always locked
• Exchanges value stored in register with value stored in memory
  • xchg src, addr
• Returns the old value, src

cmpxchg

• Always locked
• Compares two values and exchanges them if different

lfence (load fence)

• Wait until all loads before the lfence are done before continuing
  • Load instructions cannot be moved from before the fence to after the fence

sfence (store fence)

• Waits until all stores before the sfence are done before continuing
  • Store instruction cannot be moved from before the fence to after the fence

mfence (memory fence)

• Waits for loads and stores
• Combination of lfence and sfence