[SOLVED] Linux kernel: why preemption is disabled when use per-CPU variable?

Issue

Im looking at this macro from the linux kernel which has to do with handling cpu-specific variables

#define get_cpu_var(var)                                                \
(*({                                                                    \
        preempt_disable();                                              \
        this_cpu_ptr(&var);                                             \
})) 

Why do we disable preemption? Isnt preemption something that cant happen when you are in the kernel? (Since the kernel is the one doing the preemption)

Solution

Why do we disable preemption?

To avoid having the thread preempted and rescheduled on a different processor core.

Isnt preemption something that cant happen when you are in the kernel?

This was true when there was still a big kernel lock. Having one global lock means that if you block in-kernel, no other thread may enter the kernel. Now, with the more fine-grained locking, sleeping in the kernel is possible. Linux can be configured at build-time for other preemption models, e.g. CONFIG_PREEMPT.

While your normal desktop kernel is probably configured with CONFIG_PREEMPT_VOLUNTARY, some distributions also ship CONFIG_PREEMPT as a separate low-latency kernel package, e.g. for audio use. For real-time use cases, The preempt_rt patchset even makes most spinlocks preemptible (hence the name).

Answered By - a3f

Answer Checked By - Cary Denson (WPSolving Admin)