linux-kernel - Re: BUG: tracer_alloc_buffers returned with preemption imbalance

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Message-ID: <CAM_iQpW-vtnHUOBoROueui-FCrw9zB4U3vwy=PnEOLXP713WWw@mail.gmail.com>
Date:	Sun, 24 Jun 2012 16:37:28 +0800
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: BUG: tracer_alloc_buffers returned with preemption imbalance

On Sun, Jun 24, 2012 at 1:48 AM, Fengguang Wu <fengguang.wu@...el.com> wrote:
>> >  static inline int rcu_blocking_is_gp(void)
>> >  {
>> >         might_sleep();  /* Check for RCU read-side critical section. */
>> > +       preempt_disable();
>> >         return num_online_cpus() <= 1;
>> > +       preempt_enable();
>> >  }
>>
>> Thank you!  I have no idea how a preempt_disable() causes that badness
>> to happen, but this commit is not yet critically important, so I will
>> drop it.
>
> preempt_enable() becomes dead code because of the return statement?
> I wonder why gcc didn't issue a warning (or I failed to catch it)...
>

gcc has an option -Wunreachable-code, but we don't enable it
when building kernel, nor it will be enabled with -Wall. If we enable it,
we will have many false-positives as we have lots of debugging code
which is not reachable unless we enable some debugging option.

However, when I test it manually with the following code:

~% cat /tmp/unreachable.c
int main(void)
{
	int a = 0;
	a++;
	return ++a;
	a++;
}
~% gcc -Wunreachable-code -O0 -c /tmp/unreachable.c

gcc still doesn't give me any warning for the last line of the code,
gcc optimizes it out silently, I am wondering if this is a gcc bug.
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