CODEWITHSUNDEEP
multithreadinglinux-kernelinterrupt-handlingsoftirq
user2950911
user2950911

Reputation: 933

local_bh_disable, preempt_disable, local_irq_disable

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)?

Upvotes: 5

Views: 5170

Answers (2)

alex8888
alex8888

Reputation: 11

preempt_disable/enable scope ensures that calling schedule inside that scope does nothing (i.e. preemption is disabled). However, a softirq or a irq can interrupt you.

Disabling irq will only disable hard interrupts, as disabling bh(softirqs) will only disable software interrupts, but you need to explicitly specify which one you want to disable. There are 4 levels: NMI, IRQ, softirq, process. NMI(non maskable interrupts) can interrupt IRQ, softirq, process; IRQ can interrupt a softirq and a process; softirqs can interrupt a process.

Calling local_bh_disable() after local_irq_save() may be redundant (not sure), but calling local_bh_disable() after preempt_disable() is definitely needed if you want to disable BH.

Upvotes: 1

Gil Hamilton
Gil Hamilton

Reputation: 12357

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.

Upvotes: 3

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