Why empty critical section within netfilter hooks, occurs `BUG: scheduling while atomic error`?

Question

I've written this hook: ``` #include #include #include #include #include #include static struct nf_hook_ops nfho; static struct mutex critical_section; unsigned int hook_func(unsigned int hooknum, struct sk_buff **skb, const struct net_device *in, const struct net_device *out, int (*okfn)(struct sk_buff *)) { mutex_lock(&critical_section); mutex_unlock(&critical_section); return NF_ACCEPT; } int init_module() { nfho.hook = hook_func; nfho.hooknum = NF_INET_PRE_ROUTING; nfho.pf = PF_INET; nfho.priority = NF_IP_PRI_FIRST; mutex_init(&critical_section); nf_register_hook(&nfho); return 0; } void cleanup_module() { nf_unregister_hook(&nfho); } ``` init section: ``` mutex_init(&queue_critical_section); mutex_init(&ioctl_critical_section); ``` I have defined static variable: ``` static struct mutex queue_critical_section; ``` As there is no code between `lock` and `unlock` I expect…

Accepted Answer

This is a hook from inside the kernel. Sleeping, locking a semaphore (pend) or any blocking operations are not allowed; You are locking the kernel!

If you want synchronization object, you might try to use spin locks.

As this answer to similar question stated, mutex_lock will trigger scheduler; But the kernel will be puzzled because you are trying to schedule another task, while you are in critical section (the callback itself is big critical section).

Check this thread Understanding execution context of netfilter hooks for similar case.

Why empty critical section within netfilter hooks, occurs `BUG: scheduling while atomic error`?

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