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Subject: Re: [PATCH tip/core/urgent 4/7] rcu: protect __rcu_read_unlock()
 against scheduler-using irq handlers
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@polymtl.ca, josh@joshtriplett.org, niv@us.ibm.com,
        tglx@linutronix.de, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu,
        dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com,
        patches@linaro.org, greearb@candelatech.com, edt@aei.ca,
        "Paul E. McKenney" <paul.mckenney@linaro.org>
In-Reply-To: <1311121103-16978-4-git-send-email-paulmck@linux.vnet.ibm.com>
References: <20110720001738.GA16369@linux.vnet.ibm.com>
	 <1311121103-16978-4-git-send-email-paulmck@linux.vnet.ibm.com>
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Date: Wed, 20 Jul 2011 14:54:27 +0200
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On Tue, 2011-07-19 at 17:18 -0700, Paul E. McKenney wrote:
> @@ -391,10 +400,15 @@ void __rcu_read_unlock(void)
>         struct task_struct *t = current;
>  
>         barrier();  /* needed if we ever invoke rcu_read_unlock in rcutree.c */
> -       if (--t->rcu_read_lock_nesting == 0) {
> -               barrier();  /* decr before ->rcu_read_unlock_special load */
> +       if (t->rcu_read_lock_nesting != 1)
> +               --t->rcu_read_lock_nesting;
> +       else {
> +               t->rcu_read_lock_nesting = INT_MIN;
> +               barrier();  /* assign before ->rcu_read_unlock_special load */
>                 if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
>                         rcu_read_unlock_special(t);
> +               barrier();  /* ->rcu_read_unlock_special load before assign */
> +               t->rcu_read_lock_nesting = 0;
>         }
>  #ifdef CONFIG_PROVE_LOCKING
>         WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0); 

But won't the above change make that WARN_ON_ONCE() invalid?