Re: [PATCH] softirq: reorder trace_softirqs_on to prevent lockdep splat

[Steven Rostedt]

On Thu, 26 Apr 2018 21:11:00 -0700
Joel Fernandes <joelaf@xxxxxxxxxx> wrote:

> 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.
> 

OK, so 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.

Your patch makes the tracing happen before we monkey with softirqs.

Yeah, looks good to me.

Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

Thomas, you want to take this?

-- Steve


> 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@xxxxxxxxxxx>
> Cc: Peter Zilstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Tom Zanussi <tom.zanussi@xxxxxxxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Thomas Glexiner <tglx@xxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: Fenguang Wu <fengguang.wu@xxxxxxxxx>
> Cc: Baohong Liu <baohong.liu@xxxxxxxxx>
> Cc: Vedang Patel <vedang.patel@xxxxxxxxx>
> Cc: kernel-team@xxxxxxxxxxx
> Signed-off-by: Joel Fernandes <joelaf@xxxxxxxxxx>
> ---
>  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);
>  }
>  
>  /*