[tip: core/rcu] rcu: Don't set nesting depth negative in rcu_preempt_deferred_qs()

[tip-bot2 for Lai Jiangshan]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     07b4a930fc44a537efecf73c1fd2b4937f64caaa
Gitweb:        https://git.kernel.org/tip/07b4a930fc44a537efecf73c1fd2b4937f64caaa
Author:        Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
AuthorDate:    Sat, 15 Feb 2020 14:37:26 -08:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 27 Apr 2020 11:03:50 -07:00

rcu: Don't set nesting depth negative in rcu_preempt_deferred_qs()

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_preempt_deferred_qs() being invoked within such a handler
that interrupted an extended RCU read-side critical section, 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_preempt_deferred_qs() to set the
nesting level negative.

This commit therefore removes this recursion-protection code from
rcu_preempt_deferred_qs().

[ paulmck: Fix typo in commit log per Steve Rostedt. ]
Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/tree_plugin.h | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index ccad776..263c766 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -569,16 +569,11 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
 static void rcu_preempt_deferred_qs(struct task_struct *t)
 {
 	unsigned long flags;
-	bool couldrecurse = rcu_preempt_depth() >= 0;
 
 	if (!rcu_preempt_need_deferred_qs(t))
 		return;
-	if (couldrecurse)
-		rcu_preempt_depth_set(rcu_preempt_depth() - RCU_NEST_BIAS);
 	local_irq_save(flags);
 	rcu_preempt_deferred_qs_irqrestore(t, flags);
-	if (couldrecurse)
-		rcu_preempt_depth_set(rcu_preempt_depth() + RCU_NEST_BIAS);
 }
 
 /*