Linux - Locking between timer, tasklet, and kthread on SMP ARM

Question

I have some data that are shared between a tasklet, a timer, and a kthread on a SMP ARM. What is the proper type of spin lock to use?

According to Cheat Sheet For Locking, it seems like a regular spin_lock()/spin_unlock() would do the job. But as this is purely bottom halves, shouldn't spin_lock_bh() be enough?

Thanks!

jdizzle · Accepted Answer · 2016-04-10 02:02:12Z

0

You need to use spin_lock_bh here because you're sharing a data structure between a tasklet and a kthread.

Using only a spin_lock may result in a deadlock:

kthread takes lock
irq occurs
softirq runs after leaving irq context
your tasklet runs
tasklet tries to take lock

answered Apr 10, 2016 at 2:02

jdizzle

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Comments

user2867169 · Accepted Answer · 2016-04-11 07:22:31Z

spin_lock() might lead to deadlock issues. Consider a case where process acquires lock for critical section and a software interrupt arrives on same CPU which also tries to acquire the same lock which process have. This is case of deadlock where none process can move forward. Hence we require that when spinlock acquired on CPU, local software interrupts should be disabled. This is done by spin_lock_bh().

Hence when a data structure is shared between a process context and bottom half, it is secured by spin_lock_bh().

However, sharing of data structure between process context and bottom half is not recommended unless extremely necessary to do. Per-cpu variables could be a replacement to avoid locking.

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