linux-kernel - Re: [PATCH] rcu: Convert timers to use timer

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Message-Id: <20171030180446.GY3659@linux.vnet.ibm.com>
Date:   Mon, 30 Oct 2017 11:04:46 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Convert timers to use timer_setup()

On Tue, Oct 24, 2017 at 02:32:04AM -0700, Kees Cook wrote:
> In preparation for unconditionally passing the struct timer_list pointer to
> all timer callbacks, switch to using the new timer_setup() and from_timer()
> to pass the timer pointer explicitly.
> 
> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Lai Jiangshan <jiangshanlai@...il.com>
> Signed-off-by: Kees Cook <keescook@...omium.org>

One question below.

							Thanx, Paul

> ---
>  kernel/rcu/rcutorture.c  | 4 ++--
>  kernel/rcu/tree_plugin.h | 9 +++++----
>  2 files changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index e1d3fa534ac0..b6fbbeb5a7da 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -1078,7 +1078,7 @@ static void rcu_torture_timer_cb(struct rcu_head *rhp)
>   * counter in the element should never be greater than 1, otherwise, the
>   * RCU implementation is broken.
>   */
> -static void rcu_torture_timer(unsigned long unused)
> +static void rcu_torture_timer(struct timer_list *unused)
>  {
>  	int idx;
>  	unsigned long started;
> @@ -1165,7 +1165,7 @@ rcu_torture_reader(void *arg)
>  	VERBOSE_TOROUT_STRING("rcu_torture_reader task started");
>  	set_user_nice(current, MAX_NICE);
>  	if (irqreader && cur_ops->irq_capable)
> -		setup_timer_on_stack(&t, rcu_torture_timer, 0);
> +		timer_setup_on_stack(&t, rcu_torture_timer, 0);
> 
>  	do {
>  		if (irqreader && cur_ops->irq_capable) {
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 8a5a3f9b1250..5fb7beee76aa 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2265,9 +2265,11 @@ static void do_nocb_deferred_wakeup_common(struct rcu_data *rdp)
>  }
> 
>  /* Do a deferred wakeup of rcu_nocb_kthread() from a timer handler. */
> -static void do_nocb_deferred_wakeup_timer(unsigned long x)
> +static void do_nocb_deferred_wakeup_timer(struct timer_list *t)
>  {
> -	do_nocb_deferred_wakeup_common((struct rcu_data *)x);
> +	struct rcu_data *x = from_timer(x, t, nocb_timer);

As long as we are creating a real typed variable for this could we
please call it "rdp" in order to follow the usual RCU conventions?

	struct rcu_data *rdp = from_timer(rdp, t, nocb_timer);

> +
> +	do_nocb_deferred_wakeup_common(x);

And of course here:

	do_nocb_deferred_wakeup_common(rdp);

>  }
> 
>  /*
> @@ -2331,8 +2333,7 @@ static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
>  	init_swait_queue_head(&rdp->nocb_wq);
>  	rdp->nocb_follower_tail = &rdp->nocb_follower_head;
>  	raw_spin_lock_init(&rdp->nocb_lock);
> -	setup_timer(&rdp->nocb_timer, do_nocb_deferred_wakeup_timer,
> -		    (unsigned long)rdp);
> +	timer_setup(&rdp->nocb_timer, do_nocb_deferred_wakeup_timer, 0);

Shouldn't this instead be something like this, give or take casts?

	timer_setup(&rdp->nocb_timer, do_nocb_deferred_wakeup_timer, rdp);

Otherwise, I don't see how do_nocb_deferred_wakeup_common() avoids a
NULL-pointer dereference.

>  }
> 
>  /*
> -- 
> 2.7.4
> 
> 
> -- 
> Kees Cook
> Pixel Security
>