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KFunc bpf_rcu_read_lock

v6.2

Definition

This kfunc is used to define a RCU read lock region in the BPF program. The end of such a region is marked by bpf_rcu_read_unlock

The current implementation does not support nested RCU read lock region in the program.

Signature

void bpf_rcu_read_lock()

Usage

  struct task_struct {
    ...
    struct task_struct              *last_wakee;
    struct task_struct __rcu        *real_parent;
    ...
  };

Let us say the program executes task = bpf_get_current_task_btf() to get a task pointer. The basic rules are:

  • 'real_parent = task->real_parent should be inside bpf_rcu_read_lock region. This is to simulate rcu_dereference() operation. The real_parent is marked as MEM_RCU only if (1). task->real_parent is inside bpf_rcu_read_lock region, and (2). task is a trusted_ptr. So MEM_RCU marked pointer can be "trusted" inside the bpf_rcu_read_lock region.
  • last_wakee = real_parent->last_wakee should be inside bpf_rcu_read_lock region since it tries to access rcu protected memory.
  • the last_wakee pointer will be marked as PTR_UNTRUSTED since in general it is not clear whether the object pointed by last_wakee is valid or not even inside bpf_rcu_read_lock region.

Program types

The following program types can make use of this kfunc:

Example

Docs could be improved

This part of the docs is incomplete, contributions are very welcome