watchdog/hardlockup/perf: Fix perf_event memory leak

During stress-testing, we found a kmemleak report for perf_event:

  unreferenced object 0xff110001410a33e0 (size 1328):
    comm "kworker/4:11", pid 288, jiffies 4294916004
    hex dump (first 32 bytes):
      b8 be c2 3b 02 00 11 ff 22 01 00 00 00 00 ad de  ...;....".......
      f0 33 0a 41 01 00 11 ff f0 33 0a 41 01 00 11 ff  .3.A.....3.A....
    backtrace (crc 24eb7b3a):
      [<00000000e211b653>] kmem_cache_alloc_node_noprof+0x269/0x2e0
      [<000000009d0985fa>] perf_event_alloc+0x5f/0xcf0
      [<00000000084ad4a2>] perf_event_create_kernel_counter+0x38/0x1b0
      [<00000000fde96401>] hardlockup_detector_event_create+0x50/0xe0
      [<0000000051183158>] watchdog_hardlockup_enable+0x17/0x70
      [<00000000ac89727f>] softlockup_start_fn+0x15/0x40
      ...

Our stress test includes CPU online and offline cycles, and updating the
watchdog configuration.

After reading the code, I found that there may be a race between cleaning up
perf_event after updating watchdog and disabling event when the CPU goes offline:

  CPU0                          CPU1                           CPU2
  (update watchdog)                                            (hotplug offline CPU1)

  ...                                                          _cpu_down(CPU1)
  cpus_read_lock()                                             // waiting for cpu lock
    softlockup_start_all
      smp_call_on_cpu(CPU1)
                                softlockup_start_fn
                                ...
                                  watchdog_hardlockup_enable(CPU1)
                                    perf create E1
                                    watchdog_ev[CPU1] = E1
  cpus_read_unlock()
                                                               cpus_write_lock()
                                                               cpuhp_kick_ap_work(CPU1)
                                cpuhp_thread_fun
                                ...
                                  watchdog_hardlockup_disable(CPU1)
                                    watchdog_ev[CPU1] = NULL
                                    dead_event[CPU1] = E1
  __lockup_detector_cleanup
    for each dead_events_mask
      release each dead_event
      /*
       * CPU1 has not been added to
       * dead_events_mask, then E1
       * will not be released
       */
                                    CPU1 -> dead_events_mask
    cpumask_clear(&dead_events_mask)
    // dead_events_mask is cleared, E1 is leaked

In this case, the leaked perf_event E1 matches the perf_event leak
reported by kmemleak. Due to the low probability of problem recurrence
(only reported once), I added some hack delays in the code:

  static void __lockup_detector_reconfigure(void)
  {
    ...
          watchdog_hardlockup_start();
          cpus_read_unlock();
  +       mdelay(100);
          /*
           * Must be called outside the cpus locked section to prevent
           * recursive locking in the perf code.
    ...
  }

  void watchdog_hardlockup_disable(unsigned int cpu)
  {
    ...
                  perf_event_disable(event);
                  this_cpu_write(watchdog_ev, NULL);
                  this_cpu_write(dead_event, event);
  +               mdelay(100);
                  cpumask_set_cpu(smp_processor_id(), &dead_events_mask);
                  atomic_dec(&watchdog_cpus);
    ...
  }

  void hardlockup_detector_perf_cleanup(void)
  {
    ...
                          perf_event_release_kernel(event);
                  per_cpu(dead_event, cpu) = NULL;
          }
  +       mdelay(100);
          cpumask_clear(&dead_events_mask);
  }

Then, simultaneously performing CPU on/off and switching watchdog, it is
almost certain to reproduce this leak.

The problem here is that releasing perf_event is not within the CPU
hotplug read-write lock. Commit:

  941154bd6937 ("watchdog/hardlockup/perf: Prevent CPU hotplug deadlock")

introduced deferred release to solve the deadlock caused by calling
get_online_cpus() when releasing perf_event. Later, commit:

  efe951d3de91 ("perf/x86: Fix perf,x86,cpuhp deadlock")

removed the get_online_cpus() call on the perf_event release path to solve
another deadlock problem.

Therefore, it is now possible to move the release of perf_event back
into the CPU hotplug read-write lock, and release the event immediately
after disabling it.

Fixes: 941154bd6937 ("watchdog/hardlockup/perf: Prevent CPU hotplug deadlock")
Signed-off-by: Li Huafei <lihuafei1@huawei.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241021193004.308303-1-lihuafei1@huawei.com

1 files changed, 0 insertions, 5 deletions

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 07455d25329c..ad755db29efd 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1453,11 +1453,6 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
 
 out:
 	cpus_write_unlock();
-	/*
-	 * Do post unplug cleanup. This is still protected against
-	 * concurrent CPU hotplug via cpu_add_remove_lock.
-	 */
-	lockup_detector_cleanup();
 	arch_smt_update();
 	return ret;
 }