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Date: Mon, 17 Feb 2020 15:26:35 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Lai Jiangshan <laijs@...ux.alibaba.com>
Cc: linux-kernel@...r.kernel.org,
Josh Triplett <josh@...htriplett.org>,
Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: Re: [PATCH V2 4/7] rcu: don't use negative ->rcu_read_lock_nesting
On Sat, Nov 02, 2019 at 12:45:56PM +0000, Lai Jiangshan wrote:
> Negative ->rcu_read_lock_nesting was introduced to prevent
> scheduler deadlock. But now with the help of deferred qs
> mechanism, we can defer qs rather than persevere in reporting qs
> and deadlock. So negative ->rcu_read_lock_nesting is useless now
> and rcu_read_unlock() can be simplified.
>
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
I have queued this for further review and testing, thank you!
There were a few adjustments, so please see the updated patch below.
Thanx, Paul
------------------------------------------------------------------------
commit 756b5aea6df6d769a346d4b55cc66707b2d607a9
Author: Lai Jiangshan <laijs@...ux.alibaba.com>
Date: Sat Feb 15 15:23:26 2020 -0800
rcu: Don't use negative nesting depth in __rcu_read_unlock()
Now that RCU flavors have been consolidated, an RCU-preempt
rcu_read_unlock() in an interrupt or softirq handler cannot possibly
end the RCU read-side critical section. Consider the old vulnerability
involving rcu_read_unlock() being invoked within such a handler that
interrupted an __rcu_read_unlock_special(), in which a wakeup might be
invoked with a scheduler lock held. Because rcu_read_unlock_special()
no longer does wakeups in such situations, it is no longer necessary
for __rcu_read_unlock() to set the nesting level negative.
This commit therfore removes this recursion-protection code from
__rcu_read_unlock().
Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
[ paulmck: Let rcu_exp_handler() continue to call rcu_report_exp_rdp(). ]
[ paulmck: Adjust other checks given no more negative nesting. ]
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index c2b04da..72952ed 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -639,6 +639,7 @@ static void wait_rcu_exp_gp(struct work_struct *wp)
*/
static void rcu_exp_handler(void *unused)
{
+ int depth = rcu_preempt_depth();
unsigned long flags;
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
struct rcu_node *rnp = rdp->mynode;
@@ -649,7 +650,7 @@ static void rcu_exp_handler(void *unused)
* critical section. If also enabled or idle, immediately
* report the quiescent state, otherwise defer.
*/
- if (!rcu_preempt_depth()) {
+ if (!depth) {
if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
rcu_dynticks_curr_cpu_in_eqs()) {
rcu_report_exp_rdp(rdp);
@@ -673,7 +674,7 @@ static void rcu_exp_handler(void *unused)
* can have caused this quiescent state to already have been
* reported, so we really do need to check ->expmask.
*/
- if (rcu_preempt_depth() > 0) {
+ if (depth > 0) {
raw_spin_lock_irqsave_rcu_node(rnp, flags);
if (rnp->expmask & rdp->grpmask) {
rdp->exp_deferred_qs = true;
@@ -683,30 +684,8 @@ static void rcu_exp_handler(void *unused)
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 (!(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();
- }
+ // Finally, negative nesting depth should not happen.
+ WARN_ON_ONCE(1);
}
/* 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 be3d100..571b7c9 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -345,9 +345,7 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
return READ_ONCE(rnp->gp_tasks) != NULL;
}
-/* Bias and limit values for ->rcu_read_lock_nesting. */
-#define RCU_NEST_BIAS INT_MAX
-#define RCU_NEST_NMAX (-INT_MAX / 2)
+/* limit value for ->rcu_read_lock_nesting. */
#define RCU_NEST_PMAX (INT_MAX / 2)
static void rcu_preempt_read_enter(void)
@@ -355,9 +353,9 @@ static void rcu_preempt_read_enter(void)
current->rcu_read_lock_nesting++;
}
-static void rcu_preempt_read_exit(void)
+static int rcu_preempt_read_exit(void)
{
- current->rcu_read_lock_nesting--;
+ return --current->rcu_read_lock_nesting;
}
static void rcu_preempt_depth_set(int val)
@@ -390,21 +388,15 @@ void __rcu_read_unlock(void)
{
struct task_struct *t = current;
- if (rcu_preempt_depth() != 1) {
- rcu_preempt_read_exit();
- } else {
+ if (rcu_preempt_read_exit() == 0) {
barrier(); /* critical section before exit code. */
- rcu_preempt_depth_set(-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 */
- rcu_preempt_depth_set(0);
}
if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
int rrln = rcu_preempt_depth();
- WARN_ON_ONCE(rrln < 0 && rrln > RCU_NEST_NMAX);
+ WARN_ON_ONCE(rrln < 0 || rrln > RCU_NEST_PMAX);
}
}
EXPORT_SYMBOL_GPL(__rcu_read_unlock);
@@ -556,7 +548,7 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
{
return (__this_cpu_read(rcu_data.exp_deferred_qs) ||
READ_ONCE(t->rcu_read_unlock_special.s)) &&
- rcu_preempt_depth() <= 0;
+ rcu_preempt_depth() == 0;
}
/*
@@ -692,7 +684,7 @@ static void rcu_flavor_sched_clock_irq(int user)
} else if (rcu_preempt_need_deferred_qs(t)) {
rcu_preempt_deferred_qs(t); /* Report deferred QS. */
return;
- } else if (!rcu_preempt_depth()) {
+ } else if (!WARN_ON_ONCE(rcu_preempt_depth())) {
rcu_qs(); /* Report immediate QS. */
return;
}
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