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Subject[PATCH] softirq: reorder trace_softirqs_on to prevent lockdep splat
Date
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@goodmis.org>
Cc: Peter Zilstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Tom Zanussi <tom.zanussi@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Thomas Glexiner <tglx@linutronix.de>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Fenguang Wu <fengguang.wu@intel.com>
Cc: Baohong Liu <baohong.liu@intel.com>
Cc: Vedang Patel <vedang.patel@intel.com>
Cc: kernel-team@android.com
Signed-off-by: Joel Fernandes <joelaf@google.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
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