[PATCH] interrupt: Disable interrupt before modifying hardirq_disable counter

[Boqun Feng]

Currently a softirq may be pending longer then expected if the
triggering interrupt happens in-between hardirq_disable_enter() and
_local_interrupt_disable() in local_interrupt_disable():

    local_interrupt_disable():
      hardirq_disable_enter();
      <interrupt>
      ...
      __irq_exit_rcu():
        // false because hardirq_disable_count() is not 0
        if (.. && !hardirq_disable_count() && ..) {
	  invoke_softirq();
	}
      _local_interrupt_disable();

, it'll defer the softirq to the next interrupt which can be forever.

The order between hardirq_disable_enter() and _local_interrupt_disable()
is to optimize re-disabling interrupts if they are already disabled, but
as 1) local_interrupt_disable() is not widely used yet and 2) the proper
way to achieve this optimization may need fixing up the counter at
entry/exit time [1], so reverse the order for now to avoid the softirq
pending issue.

Since we are doing this, the part of saving the current state is
separated from irq disabling, and we basically do the following in
local_interrupt_disable():

    local_irq_save(flags);
    if (counter++ == 0) {
      this_cpu(local_interrupt_disable_state) = flags;
    }

Change _local_interrupt_disable() to _local_interrupt_save_state().

Link: https://lore.kernel.org/lkml/87v78wezid.ffs@fw13/ [1]
Reported-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Fixes: e901c1510e24 ("irq,spin_lock: Add counted interrupt disabling/enabling")
Signed-off-by: Boqun Feng <boqun@xxxxxxxxxx>
---
 include/linux/interrupt_rc.h | 19 ++++++++-----------
 kernel/softirq.c             | 17 ++++-------------
 2 files changed, 12 insertions(+), 24 deletions(-)

diff --git a/include/linux/interrupt_rc.h b/include/linux/interrupt_rc.h
index b9a7f05ecf42..e68e1bedba66 100644
--- a/include/linux/interrupt_rc.h
+++ b/include/linux/interrupt_rc.h
@@ -20,11 +20,8 @@
 /* Per-CPU interrupt disabling state for local_interrupt_{disable,enable}(). */
 DECLARE_PER_CPU(unsigned long, local_interrupt_disable_state);
 
-static __always_inline void __local_interrupt_disable(void)
+static __always_inline void __local_interrupt_save_state(unsigned long flags)
 {
-	unsigned long flags;
-
-	local_irq_save(flags);
 	raw_cpu_write(local_interrupt_disable_state, flags);
 }
 
@@ -36,9 +33,9 @@ static __always_inline void __local_interrupt_enable(void)
 }
 
 #ifndef INSTANTIATE_EXPORTED_INTERRUPT_DISABLE
-static __always_inline void _local_interrupt_disable(void)
+static __always_inline void _local_interrupt_save_state(unsigned long flags)
 {
-	__local_interrupt_disable();
+	__local_interrupt_save_state(flags);
 }
 
 static __always_inline void _local_interrupt_enable(void)
@@ -46,27 +43,27 @@ static __always_inline void _local_interrupt_enable(void)
 	__local_interrupt_enable();
 }
 #else
-extern void _local_interrupt_disable(void);
+extern void _local_interrupt_save_state(unsigned long flags);
 extern void _local_interrupt_enable(void);
 #endif
 
 #else /* !MODULE */
-extern void _local_interrupt_disable(void);
+extern void _local_interrupt_save_state(unsigned long flags);
 extern void _local_interrupt_enable(void);
 #endif /* !MODULE */
 
 static inline void local_interrupt_disable(void)
 {
 	int new_count;
+	unsigned long flags;
 
 	WARN_ON_ONCE(in_nmi());
 
+	local_irq_save(flags);
 	new_count = hardirq_disable_enter();
 
-	/* Interrupts can happen here, but it's OK, see __irq_exit_rcu(). */
-
 	if ((new_count & HARDIRQ_DISABLE_MASK) == HARDIRQ_DISABLE_OFFSET)
-		_local_interrupt_disable();
+		_local_interrupt_save_state(flags);
 }
 
 static inline void local_interrupt_enable(void)
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 7980a4a232f9..5d02c36c40e3 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -91,11 +91,11 @@ EXPORT_PER_CPU_SYMBOL_GPL(hardirq_context);
 
 DEFINE_PER_CPU(unsigned long, local_interrupt_disable_state);
 
-void _local_interrupt_disable(void)
+void _local_interrupt_save_state(unsigned long flags)
 {
-	__local_interrupt_disable();
+	__local_interrupt_save_state(flags);
 }
-EXPORT_SYMBOL(_local_interrupt_disable);
+EXPORT_SYMBOL(_local_interrupt_save_state);
 
 void _local_interrupt_enable(void)
 {
@@ -749,16 +749,7 @@ static inline void __irq_exit_rcu(void)
 #endif
 	account_hardirq_exit(current);
 	preempt_count_sub(HARDIRQ_OFFSET);
-	/*
-	 * Interrupts may happen between hardirq_disable_enter() and
-	 * local_irq_save() in local_interrupt_disable(), if irq_exit() invokes
-	 * softirq here, we may have a softirq handler calling
-	 * local_interrupt_disable() but it won't disable the IRQ because
-	 * hardirq disabling count is already 1, hence we need to prevent
-	 * invoking softirq when a local_interrupt_disable() is ongoing.
-	 */
-	if (!in_interrupt() && !hardirq_disable_count() &&
-	    local_softirq_pending()) {
+	if (!in_interrupt() && local_softirq_pending()) {
 		/*
 		 * If we left hrtimers unarmed, make sure to arm them now,
 		 * before enabling interrupts to run softirq.
-- 
2.50.1 (Apple Git-155)