softirq: Allow to drop the softirq-BKL lock on PREEMPT_RT

softirqs are preemptible on PREEMPT_RT. There is synchronisation between
individual sections which disable bottom halves. This in turn means that
a forced threaded interrupt cannot preempt another forced threaded
interrupt. Instead it will PI-boost the other handler and wait for its
completion.

This is required because code within a softirq section is assumed to be
non-preemptible and may expect exclusive access to per-CPU resources
such as variables or pinned timers.

Code with such expectation has been identified and updated to use
local_lock_nested_bh() for locking of the per-CPU resource. This means the
softirq lock can be removed.

Disable the softirq synchronization, but add a new config switch
CONFIG_PREEMPT_RT_NEEDS_BH_LOCK which allows to re-enable the synchronized
behavior in case that there are issues, which haven't been detected yet.

The softirq_ctrl.cnt accounting remains to let the NOHZ code know if
softirqs are currently handled.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/Kconfig.preempt

@@ -103,6 +103,19 @@ config PREEMPT_RT
 	  Select this if you are building a kernel for systems which
 	  require real-time guarantees.
 
+config PREEMPT_RT_NEEDS_BH_LOCK
+	bool "Enforce softirq synchronisation on PREEMPT_RT"
+	depends on PREEMPT_RT
+	help
+	  Enforce synchronisation across the softirqs context. On PREEMPT_RT
+	  the softirq is preemptible. This enforces the same per-CPU BLK
+	  semantic non-PREEMPT_RT builds have. This should not be needed
+	  because per-CPU locks were added to avoid the per-CPU BKL.
+
+	  This switch provides the old behaviour for testing reasons. Select
+	  this if you suspect an error with preemptible softirq and want test
+	  the old synchronized behaviour.
+
 config PREEMPT_COUNT
        bool
 

kernel/softirq.c

@@ -165,7 +165,11 @@ void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
 	/* First entry of a task into a BH disabled section? */
 	if (!current->softirq_disable_cnt) {
 		if (preemptible()) {
+			if (IS_ENABLED(CONFIG_PREEMPT_RT_NEEDS_BH_LOCK))
 				local_lock(&softirq_ctrl.lock);
+			else
+				migrate_disable();
+
 			/* Required to meet the RCU bottomhalf requirements. */
 			rcu_read_lock();
 		} else {
@@ -177,7 +181,8 @@ void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
 	 * Track the per CPU softirq disabled state. On RT this is per CPU
 	 * state to allow preemption of bottom half disabled sections.
 	 */
-	newcnt = __this_cpu_add_return(softirq_ctrl.cnt, cnt);
+	if (IS_ENABLED(CONFIG_PREEMPT_RT_NEEDS_BH_LOCK)) {
+		newcnt = this_cpu_add_return(softirq_ctrl.cnt, cnt);
 		/*
 		 * Reflect the result in the task state to prevent recursion on the
 		 * local lock and to make softirq_count() & al work.
@@ -189,30 +194,65 @@ void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
 			lockdep_softirqs_off(ip);
 			raw_local_irq_restore(flags);
 		}
+	} else {
+		bool sirq_dis = false;
+
+		if (!current->softirq_disable_cnt)
+			sirq_dis = true;
+
+		this_cpu_add(softirq_ctrl.cnt, cnt);
+		current->softirq_disable_cnt += cnt;
+		WARN_ON_ONCE(current->softirq_disable_cnt < 0);
+
+		if (IS_ENABLED(CONFIG_TRACE_IRQFLAGS) && sirq_dis) {
+			raw_local_irq_save(flags);
+			lockdep_softirqs_off(ip);
+			raw_local_irq_restore(flags);
+		}
+	}
 }
 EXPORT_SYMBOL(__local_bh_disable_ip);
 
 static void __local_bh_enable(unsigned int cnt, bool unlock)
 {
 	unsigned long flags;
+	bool sirq_en = false;
 	int newcnt;
 
+	if (IS_ENABLED(CONFIG_PREEMPT_RT_NEEDS_BH_LOCK)) {
 		DEBUG_LOCKS_WARN_ON(current->softirq_disable_cnt !=
 				    this_cpu_read(softirq_ctrl.cnt));
+		if (softirq_count() == cnt)
+			sirq_en = true;
+	} else {
+		if (current->softirq_disable_cnt == cnt)
+			sirq_en = true;
+	}
 
-	if (IS_ENABLED(CONFIG_TRACE_IRQFLAGS) && softirq_count() == cnt) {
+	if (IS_ENABLED(CONFIG_TRACE_IRQFLAGS) && sirq_en) {
 		raw_local_irq_save(flags);
 		lockdep_softirqs_on(_RET_IP_);
 		raw_local_irq_restore(flags);
 	}
 
-	newcnt = __this_cpu_sub_return(softirq_ctrl.cnt, cnt);
+	if (IS_ENABLED(CONFIG_PREEMPT_RT_NEEDS_BH_LOCK)) {
+		newcnt = this_cpu_sub_return(softirq_ctrl.cnt, cnt);
 		current->softirq_disable_cnt = newcnt;
 
 		if (!newcnt && unlock) {
 			rcu_read_unlock();
 			local_unlock(&softirq_ctrl.lock);
 		}
+	} else {
+		current->softirq_disable_cnt -= cnt;
+		this_cpu_sub(softirq_ctrl.cnt, cnt);
+		if (unlock && !current->softirq_disable_cnt) {
+			migrate_enable();
+			rcu_read_unlock();
+		} else {
+			WARN_ON_ONCE(current->softirq_disable_cnt < 0);
+		}
+	}
 }
 
 void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
@@ -228,7 +268,10 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
 	lock_map_release(&bh_lock_map);
 
 	local_irq_save(flags);
-	curcnt = __this_cpu_read(softirq_ctrl.cnt);
+	if (IS_ENABLED(CONFIG_PREEMPT_RT_NEEDS_BH_LOCK))
+		curcnt = this_cpu_read(softirq_ctrl.cnt);
+	else
+		curcnt = current->softirq_disable_cnt;
 
 	/*
 	 * If this is not reenabling soft interrupts, no point in trying to