Comment the current understanding of barriers and locking role around task snapshot.
Signed-off-by: Frederic Weisbecker <[email protected]> --- kernel/rcu/tasks.h | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-) diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index 6a9ee35a282e..05413b37dd6e 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -1738,9 +1738,21 @@ static void rcu_tasks_trace_pregp_step(struct list_head *hop) for_each_online_cpu(cpu) { rcu_read_lock(); /* - * RQ must be locked because no ordering exists/can be relied upon - * between rq->curr write and subsequent read sides. This ensures that - * further context switching tasks will see update side pre-GP accesses. + * RQ lock + smp_mb__after_spinlock() before reading rq->curr serve + * two purposes: + * + * 1) Ordering against previous tasks accesses (though already enforced + * by upcoming IPIs and post-gp synchronize_rcu()). + * + * 2) Make sure not to miss latest context switch, because no ordering + * exists/can be relied upon between rq->curr write and subsequent read + * sides. + * + * 3) Make sure subsequent context switching tasks will see update side + * pre-GP accesses. + * + * smp_mb() after reading rq->curr doesn't play a significant role and might + * be considered for removal in the future. */ t = cpu_curr_snapshot(cpu); if (rcu_tasks_trace_pertask_prep(t, true)) -- 2.44.0
