Hello David, Your idea behind this patch is cool, but I'm afraid that the implementation is incorrect. As you can see, the task_struct:rcu_users member shares the same memory area with the task_struct:rcu (the head of an RCU CB). Consequence: *violated invariant* that the reference counter will remain zero after reaching zero!!! After reaching zero the task_struct:rcu head is set, so further attempts to access the task_struct:rcu_users may lead to a non-zero value. For more information see https://lore.kernel.org/lkml/CAHk-=wjT6LG6sDaZtfeT80B9RaMP-y7RNRM4F5CX2v2Z=o8e=A@xxxxxxxxxxxxxx/ In my opinion, the decision about task_struct:rcu and task_struct:rcu_users union is very bad, but you should probably consult the memory separation with authors of the actual implementation. For now, in our project, we use the following approach: 1) get a reference to a valid task within RCU read-side with get_task_struct() 2) in the release function: 2.1) enter RCU read-side 2.2) if the task state is not TASK_DEAD: use put_task_struct() Note: In the case of a race with an exiting task it's OK to call put_task_struct(), because task_struct will be freed *after* the current RCU GP thanks to the task_struct:rcu_users mechanism. 2.3) otherwise if test_and_set(my_cb_flag): call_rcu(my_cb) Note1: With respect to the RCU CB API you should guarantee that your CB will be installed only once within a given RCU GP. For that purpose we use my_cb_flag. Note2: This code will race with the task_struct:rcu_users mechanism [delayed_put_task_struct()], but it's OK. Either the delayed_put_task_struct() or my_cb() can be the last to call final put_task_struct() after the current RCU GP. 2.4) otherwise: call put_task_struct() Note: The my_cb() is already installed, so within the current RCU GP we can invoke put_task_struct() and the ref counter of the task_struct will not reach zero. 2.5) release the RCU read-side 3) The RCU CB my_cb() should set the my_cb_flag to False and call put_task_struct(). If the release function is called within RCU read-side, the task_struct is guaranteed to remain valid until the end of the current RCU GP. Good luck, mY
