On Thu, 26 Apr 2018 21:11:00 -0700
Joel Fernandes <joe...@google.com> 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) <rost...@goodmis.org>
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 <rost...@goodmis.org>
> Cc: Peter Zilstra <pet...@infradead.org>
> Cc: Ingo Molnar <mi...@redhat.com>
> Cc: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
> Cc: Tom Zanussi <tom.zanu...@linux.intel.com>
> Cc: Namhyung Kim <namhy...@kernel.org>
> Cc: Thomas Glexiner <t...@linutronix.de>
> Cc: Boqun Feng <boqun.f...@gmail.com>
> Cc: Paul McKenney <paul...@linux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweis...@gmail.com>
> Cc: Randy Dunlap <rdun...@infradead.org>
> Cc: Masami Hiramatsu <mhira...@kernel.org>
> Cc: Fenguang Wu <fengguang...@intel.com>
> Cc: Baohong Liu <baohong....@intel.com>
> Cc: Vedang Patel <vedang.pa...@intel.com>
> Cc: kernel-t...@android.com
> Signed-off-by: Joel Fernandes <joe...@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);
> }
>
> /*
