Re: [PATCH v2] tracing: Fix race condition in kprobe initialization causing NULL pointer dereference

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Sep 29, 2025 at 11:38:08AM +0206, John Ogness wrote:

> >> Problem:
> >> 1. CPU0 executes (1) assigning tp_event->perf_events = list
> 
> smp_wmb()
> 
> >> 2. CPU0 executes (2) enabling kprobe functionality via class->reg()
> >> 3. CPU1 triggers and reaches kprobe_dispatcher
> >> 4. CPU1 checks TP_FLAG_PROFILE - condition passes (step 2 completed)
> 
> smp_rmb()
> 
> >> 5. CPU1 calls kprobe_perf_func() and crashes at (3) because
> >>    call->perf_events is still NULL
> >> 
> >> The issue: Assignment in step 1 may not be visible to CPU1 due to
> >> missing memory barriers before step 2 sets TP_FLAG_PROFILE flag.
> 
> A better explanation of the issue would be: CPU1 sees that kprobe
> functionality is enabled but does not see that perf_events has been
> assigned.
> 
> Add pairing read and write memory barriers to guarantee that if CPU1
> sees that kprobe functionality is enabled, it must also see that
> perf_events has been assigned.
> 
> Note that this could also be done more efficiently using a store_release
> when setting the flag (in step 2) and a load_acquire when loading the
> flag (in step 4).

The RELEASE+ACQUIRE is a better pattern for these cases. 

And I'll argue the barrier should be in 2 not 1, since it is 2 that sets
the flag checked in 4.  Any store before that flag might be affected,
not just the ->perf_events list.