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FAILED: patch "[PATCH] rcu/nocb: Perform deferred wake up before last idle's" failed to apply to 4.9-stable tree

PATCH [email protected] 1 participant 1 message Mar 1, 2021

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Stable branch Scheduler RCU
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The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 43789ef3f7d61aa7bed0cb2764e588fc990c30ef Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <[email protected]>
Date: Mon, 1 Feb 2021 00:05:45 +0100
Subject: [PATCH] rcu/nocb: Perform deferred wake up before last idle's
need_resched() check

Entering RCU idle mode may cause a deferred wake up of an RCU NOCB_GP kthread (rcuog) to be serviced.

Usually a local wake up happening while running the idle task is handled in one of the need_resched() checks carefully placed within the idle loop that can break to the scheduler.

Unfortunately the call to rcu_idle_enter() is already beyond the last generic need_resched() check and we may halt the CPU with a resched request unhandled, leaving the task hanging.

Fix this with splitting the rcuog wakeup handling from rcu_idle_enter() and place it before the last generic need_resched() check in the idle loop. It is then assumed that no call to call_rcu() will be performed after that in the idle loop until the CPU is put in low power mode.

Fixes 96d3fd0d315a (rcu: Break call_rcu() deadlock involving scheduler and perf)
Reported-by Paul E. McKenney <[email protected]>
Signed-off-by Frederic Weisbecker <[email protected]>
Signed-off-by Peter Zijlstra (Intel) <[email protected]>
Signed-off-by Ingo Molnar <[email protected]>
Cc [email protected]
Link https://lkml.kernel.org/r/[email protected] open here
include/linux/rcupdate.h +2 -0
index fd02c5fa60cb..36c2119de702 100644
@@ -110,8 +110,10 @@ static inline void rcu_user_exit(void) { }
 
 #ifdef CONFIG_RCU_NOCB_CPU
 void rcu_init_nohz(void);
+void rcu_nocb_flush_deferred_wakeup(void);
 #else /* #ifdef CONFIG_RCU_NOCB_CPU */
 static inline void rcu_init_nohz(void) { }
+static inline void rcu_nocb_flush_deferred_wakeup(void) { }
 #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
 
 /**
kernel/rcu/tree.c +0 -3
index 63032e5620b9..82838e93b498 100644
@@ -671,10 +671,7 @@ static noinstr void rcu_eqs_enter(bool user)
  */
 void rcu_idle_enter(void)
 {
-	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
-
 	lockdep_assert_irqs_disabled();
-	do_nocb_deferred_wakeup(rdp);
 	rcu_eqs_enter(false);
 }
 EXPORT_SYMBOL_GPL(rcu_idle_enter);
kernel/rcu/tree_plugin.h +5 -0
index 7e291ce0a1d6..d5b38c28abd1 100644
@@ -2187,6 +2187,11 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
 		do_nocb_deferred_wakeup_common(rdp);
 }
 
+void rcu_nocb_flush_deferred_wakeup(void)
+{
+	do_nocb_deferred_wakeup(this_cpu_ptr(&rcu_data));
+}
+
 void __init rcu_init_nohz(void)
 {
 	int cpu;
kernel/sched/idle.c +1 -0
index 305727ea0677..7199e6f23789 100644
@@ -285,6 +285,7 @@ static void do_idle(void)
 		}
 
 		arch_cpu_idle_enter();
+		rcu_nocb_flush_deferred_wakeup();
 
 		/*
 		 * In poll mode we reenable interrupts and spin. Also if we