[PATCH 08/11] rcu: don't use negative ->rcu_read_lock_nesting

Lai Jiangshan
2019-10-31T10:08:42Z

Negative ->rcu_read_lock_nesting was introduced to prevent
scheduler deadlock which was just prevented by deferred qs.
So negative ->rcu_read_lock_nesting is useless now and
rcu_read_unlock() can be simplified.

And negative ->rcu_read_lock_nesting is bug-prone,
it is good to kill it.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
kernel/rcu/tree_exp.h | 30 ++----------------------------
kernel/rcu/tree_plugin.h | 21 +++++----------------
2 files changed, 7 insertions(+), 44 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index c0d06bce35ea..9dcbd2734620 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -621,11 +621,11 @@ static void rcu_exp_handler(void *unused)
* report the quiescent state, otherwise defer.
*/
if (!t->rcu_read_lock_nesting) {
+ rdp->exp_deferred_qs = true;
if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
rcu_dynticks_curr_cpu_in_eqs()) {
- rcu_report_exp_rdp(rdp);
+ rcu_preempt_deferred_qs(t);
} else {
- rdp->exp_deferred_qs = true;
set_tsk_need_resched(t);
set_preempt_need_resched();
}
@@ -646,32 +646,6 @@ static void rcu_exp_handler(void *unused)
WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
return;
}
-
- /*
- * The final and least likely case is where the interrupted
- * code was just about to or just finished exiting the RCU-preempt
- * read-side critical section, and no, we can't tell which.
- * So either way, set ->deferred_qs to flag later code that
- * a quiescent state is required.
- *
- * If the CPU is fully enabled (or if some buggy RCU-preempt
- * read-side critical section is being used from idle), just
- * invoke rcu_preempt_deferred_qs() to immediately report the
- * quiescent state. We cannot use rcu_read_unlock_special()
- * because we are in an interrupt handler, which will cause that
- * function to take an early exit without doing anything.
- *
- * Otherwise, force a context switch after the CPU enables everything.
- */
- rdp->exp_deferred_qs = true;
- if (rcu_preempt_need_deferred_qs(t) &&
- (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
- WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs()))) {
- rcu_preempt_deferred_qs(t);
- } else {
- set_tsk_need_resched(t);
- set_preempt_need_resched();
- }
}

/* PREEMPTION=y, so no PREEMPTION=n expedited grace period to clean up after. */
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index dbded2b8c792..c62631c79463 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -344,8 +344,6 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
}

/* Bias and limit values for ->rcu_read_lock_nesting. */
-#define RCU_NEST_BIAS INT_MAX
-#define RCU_NEST_NMAX (-INT_MAX / 2)
#define RCU_NEST_PMAX (INT_MAX / 2)

/*
@@ -373,21 +371,15 @@ void __rcu_read_unlock(void)
{
struct task_struct *t = current;

- if (t->rcu_read_lock_nesting != 1) {
- --t->rcu_read_lock_nesting;
- } else {
+ if (--t->rcu_read_lock_nesting == 0) {
barrier(); /* critical section before exit code. */
- t->rcu_read_lock_nesting = -RCU_NEST_BIAS;
- barrier(); /* assign before ->rcu_read_unlock_special load */
if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
rcu_read_unlock_special(t);
- barrier(); /* ->rcu_read_unlock_special load before assign */
- t->rcu_read_lock_nesting = 0;
}
if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
int rrln = t->rcu_read_lock_nesting;

- WARN_ON_ONCE(rrln < 0 && rrln > RCU_NEST_NMAX);
+ WARN_ON_ONCE(rrln < 0);
}
}
EXPORT_SYMBOL_GPL(__rcu_read_unlock);
@@ -535,12 +527,9 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
*/
static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
{
- return (__this_cpu_read(rcu_data.exp_deferred_qs) &&
- (!t->rcu_read_lock_nesting ||
- t->rcu_read_lock_nesting == -RCU_NEST_BIAS))
- ||
- (READ_ONCE(t->rcu_read_unlock_special.s) &&
- t->rcu_read_lock_nesting <= 0);
+ return (__this_cpu_read(rcu_data.exp_deferred_qs) ||
+ READ_ONCE(t->rcu_read_unlock_special.s)) &&
+ !t->rcu_read_lock_nesting;
}

/*
--
2.20.1