Re: [PATCH v5 03/13] kprobes: Expose the optprobe jump window to Tasks RCU

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 22 Sep 2026 02:23:22 +0000
Josef Bacik <josef@toxicpanda.com> wrote:

> kprobe_optimizer() is the one synchronize_rcu_tasks() user that is not
> about trampoline text: it waits for tasks that were interrupted on an
> instruction boundary inside the bytes it is about to overwrite with the
> optimized jump, so that none of them resumes into the middle of the new
> instruction.  Those bytes are ordinary kernel or module text with no
> Tasks Trace reader around them, so on CONFIG_TASKS_RCU_TRAMPOLINE_READERS
> kernels the irq-exit quiescent-state check has to be told about them.
> 
> Add kprobe_in_optimized_region(), a lockless and conservative form of
> get_optimized_kprobe() that reports whether any registered kprobe lies
> within MAX_OPTIMIZED_LENGTH before the given address regardless of its
> optimization state, and have rcu_tasks_trampoline_text() consult it for
> core and module text so that a task interrupted there becomes a holdout
> rather than a quiescent event. The hash walk only runs while
> kprobe_optimizer() is actually inside its synchronize_rcu_tasks(),
> tracked by a flag it sets around the call; otherwise the check is a
> single load. That check cannot see a task that was already preempted in
> the region before the flag went up (possibly before the kprobe even
> existed), and the new grace period does not otherwise wait for a
> preempted task to run again, so before synchronize_rcu_tasks() the
> optimizer calls rcu_tasks_wait_irq_preempted() to wait until no parked
> task's recorded irq-exit preemption IP is inside such a region; its
> leading synchronize_rcu() also publishes the flag to every (interrupts-
> disabled) check in flight. The kprobe hash is RCU-protected and every
> free path waits for a grace period after unhashing, so the lockless walk
> from the irq-exit path is safe.
> 
> On other configurations the flag is set and cleared but nothing reads
> it and rcu_tasks_wait_irq_preempted() is a stub; the classic
> implementation already waits for such tasks.

Hmm, this seems a bit awkward. To solve this problem with tasks RCU is
not a straight solution. I rather like to suggest fixing kprobe_optimizer.

Since the root cause of the mutex related hang_task stall is the
kprobe_optimizer() waits RCU under locking mutexes -- text_mutex,
cpu_read_lock, and kprobe_mutex. When I introduced this, waiting
RCU grace period did not take so long time. But now it can take
longer, so it is a good chance to fix it completely.

Since the RCU is a kind of the generation control of internal state,
we can introduce multi-generation optimizing/freeing list and
RCU tasks callback for that.

Let me try to make another series to fix optprobe side.

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>