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SubjectRe: [PATCH ftrace/core] tracing: irqsoff: Account for additional preempt_disable
On Fri, Aug 10, 2018 at 5:55 AM, Masami Hiramatsu <mhiramat@kernel.org> wrote:
[..]
>>
>> >> The other way to fix this is to just use SRCU for all tracepoints.
>> >> However we can't do that because we can't use NMIs from RCU context.
>> >>
>> >> Fixes: c3bc8fd637a9 ("tracing: Centralize preemptirq tracepoints
>> >> and unify their usage")
>> >> Fixes: e6753f23d961 ("tracepoint: Make rcuidle tracepoint callers use SRCU")
>> >> Reported-by: Masami Hiramatsu <mhiramat@kernel.org>
>> >> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>> >> ---
>> >> kernel/trace/trace_irqsoff.c | 26 ++++++++++++++++++++++++++
>> >> 1 file changed, 26 insertions(+)
>> >>
>> >> diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
>> >> index 770cd30cda40..ffbf1505d5bc 100644
>> >> --- a/kernel/trace/trace_irqsoff.c
>> >> +++ b/kernel/trace/trace_irqsoff.c
>> >> @@ -603,14 +603,40 @@ static void irqsoff_tracer_stop(struct trace_array *tr)
>> >> */
>> >> static void tracer_hardirqs_on(void *none, unsigned long a0, unsigned long a1)
>> >> {
>> >
>> > To ensure this function must not be preempted even if we increment preempt
>> > count, I think you should check irq_disabled() whole this process, put below
>> > here.
>> >
>> > if (unlikely(!irq_disabled()))
>> > return;
>> >
>> > Since irq_disabled() will be checked in irq_trace() anyway, so no problem
>> > to return here when !irq_disabled().
>>
>> IRQs can never be enabled here. The trace hooks are called only after
>> disabling interrupts, or before enabling them. Right?
>>
>
> Even though, it should be verified or atleast commented on the function header.

Ok. ftrace/core has been fixed since since so this patch is outdated
and isn't needed any more, but I agree a separate patch doc comment
would help make it clear about this fact. Both IRQ and preempt
tracepoints work this way, they fire only within their critical
section.

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