linux-kernel - [PATCH] softirq: reorder trace_softirqs

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Date:   Thu, 26 Apr 2018 21:11:00 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     linux-kernel@...r.kernel.org
Cc:     Joel Fernandes <joelaf@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zilstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Glexiner <tglx@...utronix.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Fenguang Wu <fengguang.wu@...el.com>,
        Baohong Liu <baohong.liu@...el.com>,
        Vedang Patel <vedang.patel@...el.com>, kernel-team@...roid.com
Subject: [PATCH] softirq: reorder trace_softirqs_on to prevent lockdep splat

I'm able to reproduce a lockdep splat when CONFIG_PROVE_LOCKING=y and
CONFIG_PREEMPTIRQ_EVENTS=y.

$ echo 1 > /d/tracing/events/preemptirq/preempt_enable/enable
---
[   26.112609] DEBUG_LOCKS_WARN_ON(current->softirqs_enabled)
[   26.112636] WARNING: CPU: 0 PID: 118 at kernel/locking/lockdep.c:3854
[...]
[   26.144229] Call Trace:
[   26.144926]  <IRQ>
[   26.145506]  lock_acquire+0x55/0x1b0
[   26.146499]  ? __do_softirq+0x46f/0x4d9
[   26.147571]  ? __do_softirq+0x46f/0x4d9
[   26.148646]  trace_preempt_on+0x8f/0x240
[   26.149744]  ? trace_preempt_on+0x4d/0x240
[   26.150862]  ? __do_softirq+0x46f/0x4d9
[   26.151930]  preempt_count_sub+0x18a/0x1a0
[   26.152985]  __do_softirq+0x46f/0x4d9
[   26.153937]  irq_exit+0x68/0xe0
[   26.154755]  smp_apic_timer_interrupt+0x271/0x280
[   26.156056]  apic_timer_interrupt+0xf/0x20
[   26.157105]  </IRQ>

The problem is the softirqs annotation in lockdep goes out of sync with
the reality of the world that softirq is still off. This causes a
lockdep splat because the preempt_count_sub can call into a preemptoff
tracer or the trace events code, which inturn can call into lockdep
*before* softirqs are really turned back on, which can cause a softirqs
invalid annotation splat in lockdep.

The same issue was fixed in local_bh_disable_ip which has a comment so:
/*
 * The preempt tracer hooks into preempt_count_add and will break
 * lockdep because it calls back into lockdep after SOFTIRQ_OFFSET
 * is set and before current->softirq_enabled is cleared.
 * We must manually increment preempt_count here and manually
 * call the trace_preempt_off later.
 */

I have done a similar change to the local_bh_enable path to fix it.

Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Peter Zilstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Tom Zanussi <tom.zanussi@...ux.intel.com>
Cc: Namhyung Kim <namhyung@...nel.org>
Cc: Thomas Glexiner <tglx@...utronix.de>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Randy Dunlap <rdunlap@...radead.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Fenguang Wu <fengguang.wu@...el.com>
Cc: Baohong Liu <baohong.liu@...el.com>
Cc: Vedang Patel <vedang.patel@...el.com>
Cc: kernel-team@...roid.com
Signed-off-by: Joel Fernandes <joelaf@...gle.com>
---
 kernel/softirq.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 177de3640c78..8a040bcaa033 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -139,9 +139,13 @@ static void __local_bh_enable(unsigned int cnt)
 {
 	lockdep_assert_irqs_disabled();
 
+	if (preempt_count() == cnt)
+		trace_preempt_on(CALLER_ADDR0, get_lock_parent_ip());
+
 	if (softirq_count() == (cnt & SOFTIRQ_MASK))
 		trace_softirqs_on(_RET_IP_);
-	preempt_count_sub(cnt);
+
+	__preempt_count_sub(cnt);
 }
 
 /*
-- 
2.17.0.441.gb46fe60e1d-goog