Re: [PATCH bpf-next v3 8/9] bpf, trace, net: prepare CONFIG_DEBUG_INFO_BTF checks for a tristate

[ ... ]

Can this change introduce a deadlock when DEBUG_INFO_BTF=m and the
btf_vmlinux module is not yet loaded?

Before this change, with =m the preprocessor guard was false, so
_TRACE_BTF_IDS_INIT() expanded to nothing and trace_event_class.btf_ids
stayed NULL. With IS_ENABLED() the guard is now true for both =y and =m,
so built-in tracepoint classes initialize .btf_ids and create
events/<sys>/<event>/btf_ids tracefs files.

Reading that file for a built-in event goes through event_btf_ids_read()
in kernel/trace/trace_events.c:

    scoped_guard(mutex, &event_mutex) {
        ...
        ids = call->class->btf_ids;
        ...
        btf = btf_get_module_btf(mod);   /* mod == NULL for built-in */

btf_get_module_btf(NULL) calls bpf_get_btf_vmlinux(), which in the
IS_MODULE(CONFIG_DEBUG_INFO_BTF) case calls request_module("btf_vmlinux")
when the vmlinux BTF has not been loaded yet.

request_module() waits for modprobe to exit (UMH_WAIT_PROC) while
event_mutex is still held. Meanwhile, modprobe does finit_module() ->
load_module() -> prepare_coming_module(), which calls
trace_module_notify():

    mutex_lock(&event_mutex);
    mutex_lock(&trace_types_lock);

so the btf_vmlinux module load blocks on event_mutex held by the reader,
and the reader blocks waiting for modprobe.

The reader hangs until SIGKILLed, and every other module load and
event_mutex user (enabling/disabling events, kprobe/uprobe creation, etc.)
is stuck. Trigger: with DEBUG_INFO_BTF=m and btf_vmlinux not yet loaded,
`cat /sys/kernel/tracing/events/sched/sched_switch/btf_ids` (or any
syscall event's btf_ids).

Should the BTF be resolved before taking event_mutex, or should this path
return -ENOENT when the vmlinux BTF is not yet present?

[ ... ]


---
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