Accesses to task_struct structures must be either protected by RCU or by get_task_struct(). Tasks trace RCU uses these in a non-obvious combination, in conjunction with an IPI handler. This commit therefore adds comments explaining this usage. Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> --- kernel/rcu/tasks.h | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index 8536c55df5142..6a117375a62a1 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -785,7 +785,10 @@ EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread); // set that task's .need_qs flag so that task's next outermost // rcu_read_unlock_trace() will report the quiescent state (in which // case the count of readers is incremented). If both attempts fail, -// the task is added to a "holdout" list. +// the task is added to a "holdout" list. Note that IPIs are used +// to invoke trc_read_check_handler() in the context of running tasks +// in order to avoid ordering overhead on common-case shared-variable +// accessses. // rcu_tasks_trace_postscan(): // Initialize state and attempt to identify an immediate quiescent // state as above (but only for idle tasks), unblock CPU-hotplug @@ -994,6 +997,12 @@ static void trc_wait_for_one_reader(struct task_struct *t, } put_task_struct(t); + // If this task is not yet on the holdout list, then we are in + // an RCU read-side critical section. Otherwise, the invocation of + // rcu_add_holdout() that added it to the list did the necessary + // get_task_struct(). Either way, the task cannot be freed out + // from under this code. + // If currently running, send an IPI, either way, add to list. trc_add_holdout(t, bhp); if (task_curr(t) && -- 2.31.1.189.g2e36527f23
