xdp-project-bpf-examples/bpf-link-hang

• Demonstration of close() hang on bpf_link fd close

Demonstration of close() hang on bpf_link fd close

This is a minimal example to demonstrate an issue where close()'ing an fd for a bpf_link kernel structure will hang indefinitely if the system is fully loaded. This only happens on a kernel compiled with CONFIG_PREEMPT=y, but on such a kernel the issue is quite easy to trigger: Simply load up the system (with work inside the kernel; sending it a lot of network traffic works pretty well), and run the bpf-link-hang program as root. The program will hang until load is lowered…

The hang happens in synchronize_rcu_tasks(), which can be seen with this bpftrace invocation:

bpftrace -e 'kprobe:synchronize_rcu_tasks { @start = nsecs; printf("enter\n"); } kretprobe:synchronize_rcu_tasks { printf("exit after %d ms\n", (nsecs - @start) / 1000000); }'