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838 B
838 B
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); }'