linux-kernel - Re: [PATCH] tracing: Prevent RCU EQS breakage in preemptirq events

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Date:   Tue, 30 Jul 2019 10:50:36 +0900
From:   Eiichi Tsukata <devel@...ukata.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: Prevent RCU EQS breakage in preemptirq events

Thanks for comments.

On 2019/07/29 19:29, Peter Zijlstra wrote:
> On Sun, Jul 28, 2019 at 09:25:58PM -0700, Andy Lutomirski wrote:
>> On Sun, Jul 28, 2019 at 6:08 PM Eiichi Tsukata <devel@...ukata.com> wrote:
> 
>>> diff --git a/kernel/trace/trace_preemptirq.c b/kernel/trace/trace_preemptirq.c
>>> index 4d8e99fdbbbe..031b51cb94d0 100644
>>> --- a/kernel/trace/trace_preemptirq.c
>>> +++ b/kernel/trace/trace_preemptirq.c
>>> @@ -10,6 +10,7 @@
>>>  #include <linux/module.h>
>>>  #include <linux/ftrace.h>
>>>  #include <linux/kprobes.h>
>>> +#include <linux/context_tracking.h>
>>>  #include "trace.h"
>>>
>>>  #define CREATE_TRACE_POINTS
>>> @@ -49,9 +50,14 @@ NOKPROBE_SYMBOL(trace_hardirqs_off);
>>>
>>>  __visible void trace_hardirqs_on_caller(unsigned long caller_addr)
>>>  {
>>> +       enum ctx_state prev_state;
>>> +
>>>         if (this_cpu_read(tracing_irq_cpu)) {
>>> -               if (!in_nmi())
>>> +               if (!in_nmi()) {
>>> +                       prev_state = exception_enter();
>>>                         trace_irq_enable_rcuidle(CALLER_ADDR0, caller_addr);
>>> +                       exception_exit(prev_state);
>>> +               }
>>>                 tracer_hardirqs_on(CALLER_ADDR0, caller_addr);
>>>                 this_cpu_write(tracing_irq_cpu, 0);
>>>         }
>>
>> This seems a bit distressing.  Now we're going to do a whole bunch of
>> context tracking transitions for each kernel entry.  Would a better>> fix me to change trace_hardirqs_on_caller to skip the trace event if
>> the previous state was already IRQs on and, more importantly, to skip
>> tracing IRQs off if IRQs were already off?  The x86 code is very
>> careful to avoid ever having IRQs on and CONTEXT_USER at the same
>> time.
> 
> I think they already (try to) do that; see 'tracing_irq_cpu'.
> 

Or you mean something like this?
As for trace_hardirqs_off_caller:

diff --git a/kernel/trace/trace_preemptirq.c b/kernel/trace/trace_preemptirq.c
index 4d8e99fdbbbe..d39478bcf0f2 100644
--- a/kernel/trace/trace_preemptirq.c
+++ b/kernel/trace/trace_preemptirq.c
@@ -66,7 +66,7 @@ __visible void trace_hardirqs_off_caller(unsigned long caller_addr)
        if (!this_cpu_read(tracing_irq_cpu)) {
                this_cpu_write(tracing_irq_cpu, 1);
                tracer_hardirqs_off(CALLER_ADDR0, caller_addr);
-               if (!in_nmi())
+               if (!in_nmi() && !irqs_disabled())
                        trace_irq_disable_rcuidle(CALLER_ADDR0, caller_addr);
        }

Or

diff --git a/kernel/trace/trace_preemptirq.c b/kernel/trace/trace_preemptirq.c
index 4d8e99fdbbbe..e08c5c6ff2b3 100644
--- a/kernel/trace/trace_preemptirq.c
+++ b/kernel/trace/trace_preemptirq.c
@@ -66,8 +66,6 @@ __visible void trace_hardirqs_off_caller(unsigned long caller_addr)
        if (!this_cpu_read(tracing_irq_cpu)) {
                this_cpu_write(tracing_irq_cpu, 1);
                tracer_hardirqs_off(CALLER_ADDR0, caller_addr);
-               if (!in_nmi())
-                       trace_irq_disable_rcuidle(CALLER_ADDR0, caller_addr);
        }


As for trace_hardirqs_on_caller, it is called when IRQs off and CONTEXT_USER.
So even though we skipped the trace event if the previous state was already IRQs on,
we will fall into the same situation.