5.7.4. Concurrency and Race Conditions

Terms

• Concurrency (dt.: Nebenläufigkeit): two or more threads run at the same time and interact

• race condition (dt.: Wettlaufsituation): inconsistent results due to uncontrolled access to shared data

• deadlock (dt.: Blockierung): two or more processes are indefinetly waiting for each other to free access to a resource

• critical path (dt.: kritischer Weg): codesegment which accesses a shared resource

• mutual exclusion (dt. wechselseitiger Ausschluss): techniques used to protect critical paths against race conditions

Concurrency in the Linux kernel

• in old non-preemptive kernels (before 2.5):
  • interrupts

• 2.5 and newer:
  • kernel preemption
  • SMP (other CPU)
  • interrupts
  • delayed code execution (workqueues, tasklets, timers)

=> we need to protect our critical paths!

Advice:

• lock-free possible?

• ordering: always obtain multiple locks in the same order (avoids deadlocks)

• start with coarse locking and refine

• document which functions need to be called with which locks held