Re: [PATCH RFC v5 1/6] softirq: reorder trace_softirqs_on to prevent lockdep splat

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 30 Apr 2018 18:41:59 -0700
Joel Fernandes <joelaf@google.com> wrote:

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

Needs more info in the change log. It also requires that
CONFIG_DEBUG_LOCKED=y is set.

Add this to the change log:

The issue was this:

Start with: 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.

This patch should be separate (as you had it before), and needs to be
submitted now because it already causes issues. We can add:

Cc: stable@vger.kernel.org
Fixes: d59158162e032 ("tracing: Add support for preempt and irq enable/disable events")

-- Steve


> 
> $ echo 1 > /d/tracing/events/preemptirq/preempt_enable/enable
> 
> 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 24d243ef8e71..47e2f61938c0 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);
>  }
>  
>  /*