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a4fcfbee8f
The rcu_tasks_need_gpcb() determines whether or not: (1) There are callbacks needing another grace period, (2) There are callbacks ready to be invoked, and (3) It would be a good time to shrink back down to a single-CPU callback list. This third case is interesting because some other CPU might be adding new callbacks, which might suddenly make this a very bad time to be shrinking. This is currently handled by requiring call_rcu_tasks_generic() to enqueue callbacks under the protection of rcu_read_lock() and requiring rcu_tasks_need_gpcb() to wait for an RCU grace period to elapse before finalizing the transition. This works well in practice. Unfortunately, the current code assumes that a grace period whose end is detected by the poll_state_synchronize_rcu() in the second "if" condition actually ended before the earlier code counted the callbacks queued on CPUs other than CPU 0 (local variable "ncbsnz"). Given the current code, it is possible that a long-delayed call_rcu_tasks_generic() invocation will queue a callback on a non-zero CPU after these CPUs have had their callbacks counted and zero has been stored to ncbsnz. Such a callback would trigger the WARN_ON_ONCE() in the second "if" statement. To see this, consider the following sequence of events: o CPU 0 invokes rcu_tasks_one_gp(), and counts fewer than rcu_task_collapse_lim callbacks. It sees at least one callback queued on some other CPU, thus setting ncbsnz to a non-zero value. o CPU 1 invokes call_rcu_tasks_generic() and loads 42 from ->percpu_enqueue_lim. It therefore decides to enqueue its callback onto CPU 1's callback list, but is delayed. o CPU 0 sees the rcu_task_cb_adjust is non-zero and that the number of callbacks does not exceed rcu_task_collapse_lim. It therefore checks percpu_enqueue_lim, and sees that its value is greater than the value one. CPU 0 therefore starts the shift back to a single callback list. It sets ->percpu_enqueue_lim to 1, but CPU 1 has already read the old value of 42. It also gets a grace-period state value from get_state_synchronize_rcu(). o CPU 0 sees that ncbsnz is non-zero in its second "if" statement, so it declines to finalize the shrink operation. o CPU 0 again invokes rcu_tasks_one_gp(), and counts fewer than rcu_task_collapse_lim callbacks. It also sees that there are no callback queued on any other CPU, and thus sets ncbsnz to zero. o CPU 1 resumes execution and enqueues its callback onto its own list. This invalidates the value of ncbsnz. o CPU 0 sees the rcu_task_cb_adjust is non-zero and that the number of callbacks does not exceed rcu_task_collapse_lim. It therefore checks percpu_enqueue_lim, but sees that its value is already unity. It therefore does not get a new grace-period state value. o CPU 0 sees that rcu_task_cb_adjust is non-zero, ncbsnz is zero, and that poll_state_synchronize_rcu() says that the grace period has completed. it therefore finalizes the shrink operation, setting ->percpu_dequeue_lim to the value one. o CPU 0 does a debug check, scanning the other CPUs' callback lists. It sees that CPU 1's list has a callback, so it (rightly) triggers the WARN_ON_ONCE(). After all, the new value of ->percpu_dequeue_lim says to not bother looking at CPU 1's callback list, which means that this callback will never be invoked. This can result in hangs and maybe even OOMs. Based on long experience with rcutorture, this is an extremely low-probability race condition, but it really can happen, especially in preemptible kernels or within guest OSes. This commit therefore checks for completion of the grace period before counting callbacks. With this change, in the above failure scenario CPU 0 would know not to prematurely end the shrink operation because the grace period would not have completed before the count operation started. [ paulmck: Adjust grace-period end rather than adding RCU reader. ] [ paulmck: Avoid spurious WARN_ON_ONCE() with ->percpu_dequeue_lim check. ] Signed-off-by: Zqiang <qiang1.zhang@intel.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> |
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