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linuxkernelspinlocksoftirq

Can the SoftIRQ prempt currently running the same SoftIRQ with Linux kernel?

I am working on a performance improvement of a driver and should consider the possibility of deadlock. In a SoftIRQ context, spin_lock will be held and protect some variable. In this case, should I use spin_lock or spin_lock_bh? spin_lock_bh sounds like safer, but I have a general question about SoftIRQ implementation.

  • the same softIRQ function can be scheduled on the same CPU during the execution of the SoftIRQ -> we need to use spin_lock_bh in the SoftIRQ
  • the same softIRQ function can be scheduled on the other CPU, but not the same CPU -> we may use spin_lock in the SoftIRQ

What is true from the above statement?

Solution

  • From Linux Kernel Development (2nd edition):

    A softirq never preempts another softirq.

    The reason is simple: during softIRQ software interrupts are disabled.

    So it is sufficient to use plain spin_lock() in a softIRQ function.