Understanding the Linux Kernel, 3rd Edition

Synchronization Primitives

We now examine how kernel control paths can be interleaved while avoiding race conditions among shared data. Table 5-2 lists the synchronization techniques used by the Linux kernel. The “Scope” column indicates whether the synchronization technique applies to all CPUs in the system or to a single CPU. For instance, local interrupt disabling applies to just one CPU (other CPUs in the system are not affected); conversely, an atomic operation affects all CPUs in the system (atomic operations on several CPUs cannot interleave while accessing the same data structure).

Table 5-2. Various types of synchronization techniques used by the kernel

Technique	Description	Scope
Per-CPU variables	Duplicate a data structure among the CPUs	All CPUs
Atomic operation	Atomic read-modify-write instruction to a counter	All CPUs
Memory barrier	Avoid instruction reordering	Local CPU or All CPUs
Spin lock	Lock with busy wait	All CPUs
Semaphore	Lock with blocking wait (sleep)	All CPUs
Seqlocks	Lock based on an access counter	All CPUs
Local interrupt disabling	Forbid interrupt handling on a single CPU	Local CPU
Local softirq disabling	Forbid deferrable function handling on a single CPU	Local CPU
Read-copy-update (RCU)	Lock-free access to shared data structures through pointers	All CPUs

Let’s now briefly discuss each synchronization technique. In the later section "Synchronizing Accesses to Kernel Data Structures,” we show how these synchronization techniques can be combined to effectively protect kernel data ...