linux-kernel - [tip: locking/core] lockdep: Only trace IRQ edges

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Date:   Thu, 27 Aug 2020 07:54:24 -0000
From:   "tip-bot2 for Nicholas Piggin" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Nicholas Piggin <npiggin@...il.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Marco Elver <elver@...gle.com>, x86 <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [tip: locking/core] lockdep: Only trace IRQ edges

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

Commit-ID:     044d0d6de9f50192f9697583504a382347ee95ca
Gitweb:        https://git.kernel.org/tip/044d0d6de9f50192f9697583504a382347ee95ca
Author:        Nicholas Piggin <npiggin@...il.com>
AuthorDate:    Thu, 23 Jul 2020 20:56:14 +10:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Wed, 26 Aug 2020 12:41:56 +02:00

lockdep: Only trace IRQ edges

Problem:

  raw_local_irq_save(); // software state on
  local_irq_save(); // software state off
  ...
  local_irq_restore(); // software state still off, because we don't enable IRQs
  raw_local_irq_restore(); // software state still off, *whoopsie*

existing instances:

 - lock_acquire()
     raw_local_irq_save()
     __lock_acquire()
       arch_spin_lock(&graph_lock)
         pv_wait() := kvm_wait() (same or worse for Xen/HyperV)
           local_irq_save()

 - trace_clock_global()
     raw_local_irq_save()
     arch_spin_lock()
       pv_wait() := kvm_wait()
	 local_irq_save()

 - apic_retrigger_irq()
     raw_local_irq_save()
     apic->send_IPI() := default_send_IPI_single_phys()
       local_irq_save()

Possible solutions:

 A) make it work by enabling the tracing inside raw_*()
 B) make it work by keeping tracing disabled inside raw_*()
 C) call it broken and clean it up now

Now, given that the only reason to use the raw_* variant is because you don't
want tracing. Therefore A) seems like a weird option (although it can be done).
C) is tempting, but OTOH it ends up converting a _lot_ of code to raw just
because there is one raw user, this strips the validation/tracing off for all
the other users.

So we pick B) and declare any code that ends up doing:

	raw_local_irq_save()
	local_irq_save()
	lockdep_assert_irqs_disabled();

broken. AFAICT this problem has existed forever, the only reason it came
up is because commit: 859d069ee1dd ("lockdep: Prepare for NMI IRQ
state tracking") changed IRQ tracing vs lockdep recursion and the
first instance is fairly common, the other cases hardly ever happen.

Signed-off-by: Nicholas Piggin <npiggin@...il.com>
[rewrote changelog]
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
Tested-by: Marco Elver <elver@...gle.com>
Link: https://lkml.kernel.org/r/20200723105615.1268126-1-npiggin@gmail.com
---
 arch/powerpc/include/asm/hw_irq.h | 11 ++++-------
 include/linux/irqflags.h          | 15 +++++++--------
 2 files changed, 11 insertions(+), 15 deletions(-)

diff --git a/arch/powerpc/include/asm/hw_irq.h b/arch/powerpc/include/asm/hw_irq.h
index 3a0db7b..35060be 100644
--- a/arch/powerpc/include/asm/hw_irq.h
+++ b/arch/powerpc/include/asm/hw_irq.h
@@ -200,17 +200,14 @@ static inline bool arch_irqs_disabled(void)
 #define powerpc_local_irq_pmu_save(flags)			\
 	 do {							\
 		raw_local_irq_pmu_save(flags);			\
-		trace_hardirqs_off();				\
+		if (!raw_irqs_disabled_flags(flags))		\
+			trace_hardirqs_off();			\
 	} while(0)
 #define powerpc_local_irq_pmu_restore(flags)			\
 	do {							\
-		if (raw_irqs_disabled_flags(flags)) {		\
-			raw_local_irq_pmu_restore(flags);	\
-			trace_hardirqs_off();			\
-		} else {					\
+		if (!raw_irqs_disabled_flags(flags))		\
 			trace_hardirqs_on();			\
-			raw_local_irq_pmu_restore(flags);	\
-		}						\
+		raw_local_irq_pmu_restore(flags);		\
 	} while(0)
 #else
 #define powerpc_local_irq_pmu_save(flags)			\
diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
index 00d553d..3ed4e87 100644
--- a/include/linux/irqflags.h
+++ b/include/linux/irqflags.h
@@ -191,25 +191,24 @@ do {						\
 
 #define local_irq_disable()				\
 	do {						\
+		bool was_disabled = raw_irqs_disabled();\
 		raw_local_irq_disable();		\
-		trace_hardirqs_off();			\
+		if (!was_disabled)			\
+			trace_hardirqs_off();		\
 	} while (0)
 
 #define local_irq_save(flags)				\
 	do {						\
 		raw_local_irq_save(flags);		\
-		trace_hardirqs_off();			\
+		if (!raw_irqs_disabled_flags(flags))	\
+			trace_hardirqs_off();		\
 	} while (0)
 
 #define local_irq_restore(flags)			\
 	do {						\
-		if (raw_irqs_disabled_flags(flags)) {	\
-			raw_local_irq_restore(flags);	\
-			trace_hardirqs_off();		\
-		} else {				\
+		if (!raw_irqs_disabled_flags(flags))	\
 			trace_hardirqs_on();		\
-			raw_local_irq_restore(flags);	\
-		}					\
+		raw_local_irq_restore(flags);		\
 	} while (0)
 
 #define safe_halt()				\