On Thu, 31 May 2018 21:52:15 -0700 Joel Fernandes joel@joelfernandes.org wrote:
On Mon, May 07, 2018 at 02:21:17PM -0400, Steven Rostedt wrote:
Peter, Ingo or Thomas,
Can you queue this up? Or would you prefer that I take it?
This patch is a bug fix, could it be queued for v4.17 -rc cycle or for linux-next?
I have also included it below again:
Thanks. I can't pull it without an ack as I'm not the softirq maintainer.
Thomas, can you just ack it, and I'll take it, if that will be easire for you?
-- Steve
thanks,
- Joel
---8<-----------------------
From: "Joel Fernandes (Google)" joel@joelfernandes.org Date: Tue, 1 May 2018 18:44:39 -0700 Subject: [PATCH] softirq: reorder trace_softirqs_on to prevent lockdep splat
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: 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);
} /*