| Message ID | 20220620064224.2630382-1-qiang1.zhang@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | rcu: Update rcu_preempt_deferred_qs() comments for no preemptible RCU | expand |
On Mon, Jun 20, 2022 at 02:42:24PM +0800, Zqiang wrote: > For no preemptible RCU, the rcu_preempt_deferred_qs() only report > expedited QS state of current CPU, since preemption does not occur > in RCU critical sections, there are no tasks insert to leaf rnp > blocked-tasks list, that is to say the tasks structure's ->rcu_blocked_node > and the leaf rnp structure's ->blkd_tasks are always empty. > > Signed-off-by: Zqiang <qiang1.zhang@intel.com> Good catch -- that comment is quite misleading. Is the wordsmithed version shown below OK? Thanx, Paul ------------------------------------------------------------------------ commit 5ddf65777c8d0f60ff9137eef3b23dd4c77e6108 Author: Zqiang <qiang1.zhang@intel.com> Date: Mon Jun 20 14:42:24 2022 +0800 rcu: Update rcu_preempt_deferred_qs() comments for !PREEMPT kernels In non-premptible kernels, tasks never do context switches within RCU read-side critical sections. Therefore, in such kernels, each leaf rcu_node structure's ->blkd_tasks list will always be empty. The comment on the non-preemptible version of rcu_preempt_deferred_qs() confuses this point, so this commit therefore fixes it. Signed-off-by: Zqiang <qiang1.zhang@intel.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 86772c95ed0ae..4152816dd29f6 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -932,10 +932,13 @@ static notrace bool rcu_preempt_need_deferred_qs(struct task_struct *t) return false; } -// Except that we do need to respond to a request by an expedited grace -// period for a quiescent state from this CPU. Note that requests from -// tasks are handled when removing the task from the blocked-tasks list -// below. +// Except that we do need to respond to a request by an expedited +// grace period for a quiescent state from this CPU. Note that in +// non-preemptible kernels, there can be no context switches within RCU +// read-side critical sections, which in turn means that the leaf rcu_node +// structure's blocked-tasks list is always empty. is therefore no need to +// actually check it. Instead, a quiescent state from this CPU suffices, +// and this function is only called from such a quiescent state. notrace void rcu_preempt_deferred_qs(struct task_struct *t) { struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index dc78726b993f..99424c2da5db 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -932,10 +932,12 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t) return false; } -// Except that we do need to respond to a request by an expedited grace -// period for a quiescent state from this CPU. Note that requests from -// tasks are handled when removing the task from the blocked-tasks list -// below. +/* + * Except that we do need to respond to a request by an expedited grace + * period for a quiescent state from this CPU. Note that for no preemptible + * RCU, since preemption does not occur in RCU critical sections, so the + * leaf rnp's blocked-tasks list is always empty. + */ void rcu_preempt_deferred_qs(struct task_struct *t) { struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
For no preemptible RCU, the rcu_preempt_deferred_qs() only report expedited QS state of current CPU, since preemption does not occur in RCU critical sections, there are no tasks insert to leaf rnp blocked-tasks list, that is to say the tasks structure's ->rcu_blocked_node and the leaf rnp structure's ->blkd_tasks are always empty. Signed-off-by: Zqiang <qiang1.zhang@intel.com> --- kernel/rcu/tree_plugin.h | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-)