This is a note to let you know that I've just added the patch titled
rcu-tasks: Add trc_inspect_reader() checks for exiting critical section
to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
rcu-tasks-add-trc_inspect_reader-checks-for-exiting-critical-section.patch
and it can be found in the queue-5.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 18f08e758f34e6dfe0668bee51bd2af7adacf381 Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Date: Wed, 28 Jul 2021 11:32:28 -0700
Subject: rcu-tasks: Add trc_inspect_reader() checks for exiting critical section
From: Paul E. McKenney <paulmck@xxxxxxxxxx>
commit 18f08e758f34e6dfe0668bee51bd2af7adacf381 upstream.
Currently, trc_inspect_reader() treats a task exiting its RCU Tasks
Trace read-side critical section the same as being within that critical
section. However, this can fail because that task might have already
checked its .need_qs field, which means that it might never decrement
the all-important trc_n_readers_need_end counter. Of course, for that
to happen, the task would need to never again execute an RCU Tasks Trace
read-side critical section, but this really could happen if the system's
last trampoline was removed. Note that exit from such a critical section
cannot be treated as a quiescent state due to the possibility of nested
critical sections. This means that if trc_inspect_reader() sees a
negative nesting value, it must set up to try again later.
This commit therefore ignores tasks that are exiting their RCU Tasks
Trace read-side critical sections so that they will be rechecked later.
[ paulmck: Apply feedback from Neeraj Upadhyay and Boqun Feng. ]
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/rcu/tasks.h | 18 +++++++++---------
1 file changed, 9 insertions(+), 9 deletions(-)
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -951,7 +951,7 @@ reset_ipi:
static bool trc_inspect_reader(struct task_struct *t, void *arg)
{
int cpu = task_cpu(t);
- bool in_qs = false;
+ int nesting;
bool ofl = cpu_is_offline(cpu);
if (task_curr(t)) {
@@ -971,18 +971,18 @@ static bool trc_inspect_reader(struct ta
n_heavy_reader_updates++;
if (ofl)
n_heavy_reader_ofl_updates++;
- in_qs = true;
+ nesting = 0;
} else {
// The task is not running, so C-language access is safe.
- in_qs = likely(!t->trc_reader_nesting);
+ nesting = t->trc_reader_nesting;
}
- // Mark as checked so that the grace-period kthread will
- // remove it from the holdout list.
- t->trc_reader_checked = true;
-
- if (in_qs)
- return true; // Already in quiescent state, done!!!
+ // If not exiting a read-side critical section, mark as checked
+ // so that the grace-period kthread will remove it from the
+ // holdout list.
+ t->trc_reader_checked = nesting >= 0;
+ if (nesting <= 0)
+ return !nesting; // If in QS, done, otherwise try again later.
// The task is in a read-side critical section, so set up its
// state so that it will awaken the grace-period kthread upon exit
Patches currently in stable-queue which might be from paulmck@xxxxxxxxxx are
queue-5.15/rcu-tasks-fix-ipi-failure-handling-in-trc_wait_for_one_reader.patch
queue-5.15/rcu-tasks-add-trc_inspect_reader-checks-for-exiting-critical-section.patch
queue-5.15/rcu-tasks-wait-for-trc_read_check_handler-ipis.patch
queue-5.15/rcu-prevent-expedited-gp-from-enabling-tick-on-offline-cpu.patch
