kprobes: Make optprobe optimizer multi-generational and asynchronous

Currently, kprobe_optimizer() holds kprobe_mutex, text_mutex, and
cpus_read_lock() simultaneously while executing synchronize_rcu_tasks().
Under PREEMPT_LAZY and server workloads with long-running CPU-bound kernel
tasks or heavy cgroup writeback loops, synchronize_rcu_tasks() can block
for seconds to minutes. Because text_mutex and cpus_read_lock() are held
during this entire wait, any concurrent static key updates, module
loading/unloading, CPU hotplug, or tracing updates stall, frequently
triggering hung-task detector panics.

Instead of penalizing the kernel preemption path with invasive hooks and
global hash lookups, decouple kprobe jump optimization from synchronous
waiting entirely:

1. Replace synchronize_rcu_tasks() with call_rcu_tasks(). Locks
   (text_mutex and cpus_read_lock()) are held only for the brief moment
   needed to patch instructions via arch_unoptimize_kprobes() and
   arch_optimize_kprobes() (microseconds), and are completely released
   while waiting for the Tasks RCU grace period.

2. Introduce a fixed ring of generations (optprobe_gens[OPTPROBE_GEN_MAX])
   to avoid any dynamic memory allocation (kmalloc) or -ENOMEM failure
   modes.

3. The last generation slot in the array is reserved as a "waiting room"
   and is not dispatched to RCU until another in-flight generation has
   finished. While a generation is waiting for its Tasks RCU grace period,
   any newly registered or unregistered probes accumulate in the waiting
   room generation without blocking or requiring additional slots.

4. When the Tasks RCU callback fires, it marks the generation as ready
   and wakes up the optimizer thread to finalize optimization (poking
   the jump instructions) and free cleaned probe slots. Once finalized,
   the generation is marked idle, allowing the waiting room generation
   to be dispatched next.

5. Flushing via wait_for_kprobe_optimizer() waits asynchronously for all
   in-flight and queued generations to drain without stalling other
   kernel subsystems.

On non-preemptive kernels or configs where CONFIG_TASKS_RCU=n,
call_rcu_tasks() transparently aliases to call_rcu(), preserving full
portability.

Link: https://lore.kernel.org/all/179091652953.526694.3402334249215311330.stgit@devnote2/

Assisted-by: LLM
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

0 files changed, 0 insertions, 0 deletions