* Joel Fernandes joelaf@google.com wrote:

From: "Joel Fernandes (Google)" joel@joelfernandes.org

I'm able to reproduce a lockdep splat with config options: CONFIG_PROVE_LOCKING=y, CONFIG_DEBUG_LOCK_ALLOC=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 issue was this:

preempt_count = 1 << SOFTIRQ_SHIFT

__local_bh_enable(cnt = 1 << SOFTIRQ_SHIFT) { if (softirq_count() == (cnt && SOFTIRQ_MASK)) { trace_softirqs_on() { current->softirqs_enabled = 1; } } preempt_count_sub(cnt) { trace_preempt_on() { tracepoint() { rcu_read_lock_sched() { // jumps into lockdep

Where preempt_count still has softirqs disabled, but current->softirqs_enabled is true, and we get a splat.

Cc: Steven Rostedt rostedt@goodmis.org Cc: Peter Zijlstra peterz@infradead.org Cc: Ingo Molnar mingo@redhat.com Cc: Linus Torvalds torvalds@linux-foundation.org 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: Masami Hiramatsu mhiramat@kernel.org Cc: Todd Kjos tkjos@google.com Cc: Erick Reyes erickreyes@google.com Cc: Julia Cartwright julia@ni.com Cc: Byungchul Park byungchul.park@lge.com Cc: stable@vger.kernel.org Reviewed-by: Steven Rostedt (VMware) rostedt@goodmis.org Fixes: d59158162e032 ("tracing: Add support for preempt and irq enable/disable events") Signed-off-by: Joel Fernandes (Google) joel@joelfernandes.org

---

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);

}

Acked-by: Ingo Molnar mingo@kernel.org

Steve, wanna queue this up? Splat should only occur with tracing AFAICT.

Thanks,

Ingo