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task_css_set_check() uses rcu_dereference_check() to verify that task->cgroups can be dereferenced. One accepted condition is that the task is already exiting, tested by checking PF_EXITING in task->flags. This check is only part of the CONFIG_PROVE_RCU lockdep predicate. This was found by KCSAN during fuzz testing. KCSAN can report a data race when another task flag bit is updated concurrently. One report shows pids_release() reading task->flags through task_css_set_check() while do_task_dead() sets PF_NOFREEZE: KCSAN: data-race in task_css() [inline] KCSAN: data-race in pids_release() task_css() pids_release() cgroup_release() release_task() wait_task_zombie() value changed: 0x0040004c -> 0x0040804c The changed bit is PF_NOFREEZE, not PF_EXITING. PF_EXITING remains set before and after the update, so the task_css_set_check() condition does not change. This is not a race on task->cgroups and does not indicate incorrect pids charging or uncharging. tools/memory-model/Documentation/access-marking.txt recommends data_race() for data-racy loads used only for diagnostic purposes. Use data_race() here to mark the intended diagnostic-only access. No functional change intended. Suggested-by: Tejun Heo <tj@kernel.org> Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn> Signed-off-by: Tejun Heo <tj@kernel.org> |
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