> What does the above mean? Are you loading two modules at the same time?
Two modules (A and B) are loaded simultaneously on different CPUs. On the arm64,
when CPU0's trace_module_notify [pri=1] and CPU1's trace_module_notify [pri=0]
simultaneously perform operations on call_A, because they are in different
cache lines,
CPU1 may observe WRITE_ONCE(head->next, &f->link) in step (4) before
f->link.next=next in step (2).
At this time, CPU1 reads an uninitialized f->link.next and performs an
operation that causes to crash.
> What does "pri=X notifier" mean? What function calls are these coming from?
`pri=X notifier` represents `trace_events.c:trace_module_notify [pri=1]` and
`trace.c:trace_module_notify [pri=0]`, respectively.
CPU0 (loads module A) CPU1 (loads module B)
=============================== ===============================
load_module(A) load_module(B)
blocking_notifier_call_chain_robust
blocking_notifier_call_chain_robust
notifier_call_chain notifier_call_chain
nb = trace_events.c: nb = trace.c:
trace_module_notify [pri=1] trace_module_notify [pri=0]
mutex_lock(&event_mutex) trace_event_update_all()
trace_module_add_events(A)
down_write(&trace_event_sem)
__register_event(call_A)
__add_event_to_tracers(call_A)
event_define_fields(call_A)
for each f:
f = kmem_cache_alloc()
list_add(&f->link,
&class->fields)
f->link.next=next; (2)
WRITE_ONCE(head->next,
&f->link); (4)
update_event_fields(call_A)
mutex_unlock(&event_mutex)
list_for_each_entry(field,
&class->fields, link)
field =
class->fields->next
= &f->link
= f (offset 0)
up_write(&trace_event_sem)
On 8/10/2026 10:45 PM, Steven Rostedt wrote:
> On Mon, 10 Aug 2026 14:32:30 +0800
> Michael Wu <[email protected]> wrote:
>
>> The following sequence may leads race between event_define_fields()
>> and update_event_fields():
>
>> CPU0 (module A, pri=1 notifier) CPU1 (module B, pri=0 notifier)
>
> What does the above mean? Are you loading two modules at the same time?
> What does "pri=X notifier" mean? What function calls are these coming from?
>
> -- Steve
>
>> =============================== ===============================
>> event_define_fields(call_A) trace_event_update_all()
>> for each f: list_for_each_entry(...,
>> list_add(&f->link, &ftrace_events)
>> &class->fields) -> finds call_A
>> f->link.next = next; (2)
>> update_event_fields(call_A)
>> WRITE_ONCE(class->fields->next,
>> &f->link); (4)
>> list_for_each_entry(field,
>> &class->fields, link)
>> -> field = class->fields->next
>> = &f->link
>> = f (offset 0)
>> -> arm64 weak ordering:
>> (4) visible before (2)
>> field->link.next == 0
>> -> next iteration:
>> field = (void *)0 = NULL
>> -> crash at NULL->type (0x18)
>>
>> This produces the following panic:
>> Unable to handle kernel access ... at virtual address 0000000000000018
>> pc : update_event_fields+0xf8/0x368
>> Call trace:
>> update_event_fields+0xf8/0x368
>> trace_event_update_all+0x7c/0x2b4
>> trace_module_notify+0x4c/0x1dc
>> notifier_call_chain+0x84/0x168
>> blocking_notifier_call_chain_robust+0x64/0xd4
>> load_module+0x10c8/0x123c
>> __arm64_sys_finit_module+0x230/0x31c
>>
>> Fix by taking event_mutex in trace_event_update_all() before
>> trace_event_sem.
>>
>> Fixes: b3bc8547d3be ("tracing: Have TRACE_DEFINE_ENUM affect trace event
>> types as well")
>> Cc: [email protected]
>> Signed-off-by: Michael Wu <[email protected]>
--
Regards,
Michael Wu
