local_bh_disable, preempt_disable, local_irq_disable

Question

local_bh_disable disables the processing of bottom halves (softirqs). Softirqs are processed on either, interrupt return path, or by the ksoftirqd-(per cpu)-thread that will be woken up if the system suffers of heavy softirq-load.

preempt_disable disables preemption, which means, that while a thread is executing inside a preempt_disable <-> preemt_enable scope, it will not be put to sleep by the scheduler. This means that, if the system-timer-interrupt occurs while the current thread is inside that scope, it might update the accouting tables of the scheduler, but it will not switch context to another thread. this includes the softirqd.

local_irq_disable or local_irq_save disable interrupts for the local cpu. this means that the local cpu will not react to any irqs, so it will not run any interrupt return paths and hence, cannot run softirqs there.

Assuming my above statements are true (which i am not sure about), then wouldnt it be redundant to call local_bh_disable AFTER you called preempt_disable and local_irq_save (while being in process context)?

Answer 1

Yes. Once local_irq_save / disable has been called, no further protection is needed -- you won't be interrupted (except by an NMI or an exception in your code).

Often, however, you'll find bits of code that are designed to be callable from different contexts, so they may provide protection for some sub-operation that ends up being redundant in some paths.