linux-kernel - Re: [PATCH tip/core/rcu 10/10] cpu: Avoid puts

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Message-ID: <CAJhHMCDwxmNS0vA_9LqRKHxOVo=4RY-oHt2mPaVQK7MZJQ97=w@mail.gmail.com>
Date:	Wed, 29 Oct 2014 13:56:01 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>, dvhart@...ux.intel.com,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH tip/core/rcu 10/10] cpu: Avoid puts_pending overflow

On Tue, Oct 28, 2014 at 5:53 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
>
> A long string of get_online_cpus() with each followed by a
> put_online_cpu() that fails to acquire cpu_hotplug.lock can result in
> overflow of the cpu_hotplug.puts_pending counter.  Although this is
> perhaps improbably, a system with absolutely no CPU-hotplug operations
> will have an arbitrarily long time in which this overflow could occur.
> This commit therefore adds overflow checks to get_online_cpus() and
> try_get_online_cpus().
>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

This patch seems to be missing in the cover letter.

Reviewed-by: Pranith Kumar <bobby.prani@...il.com>

> ---
>  kernel/cpu.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 90a3d017b90c..5d220234b3ca 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -86,6 +86,16 @@ static struct {
>  #define cpuhp_lock_acquire()      lock_map_acquire(&cpu_hotplug.dep_map)
>  #define cpuhp_lock_release()      lock_map_release(&cpu_hotplug.dep_map)
>
> +static void apply_puts_pending(int max)
> +{
> +       int delta;
> +
> +       if (atomic_read(&cpu_hotplug.puts_pending) >= max) {
> +               delta = atomic_xchg(&cpu_hotplug.puts_pending, 0);
> +               cpu_hotplug.refcount -= delta;
> +       }
> +}
> +
>  void get_online_cpus(void)
>  {
>         might_sleep();
> @@ -93,6 +103,7 @@ void get_online_cpus(void)
>                 return;
>         cpuhp_lock_acquire_read();
>         mutex_lock(&cpu_hotplug.lock);
> +       apply_puts_pending(65536);
>         cpu_hotplug.refcount++;
>         mutex_unlock(&cpu_hotplug.lock);
>  }
> @@ -105,6 +116,7 @@ bool try_get_online_cpus(void)
>         if (!mutex_trylock(&cpu_hotplug.lock))
>                 return false;
>         cpuhp_lock_acquire_tryread();
> +       apply_puts_pending(65536);
>         cpu_hotplug.refcount++;
>         mutex_unlock(&cpu_hotplug.lock);
>         return true;
> @@ -161,12 +173,7 @@ void cpu_hotplug_begin(void)
>         cpuhp_lock_acquire();
>         for (;;) {
>                 mutex_lock(&cpu_hotplug.lock);
> -               if (atomic_read(&cpu_hotplug.puts_pending)) {
> -                       int delta;
> -
> -                       delta = atomic_xchg(&cpu_hotplug.puts_pending, 0);
> -                       cpu_hotplug.refcount -= delta;
> -               }
> +               apply_puts_pending(1);
>                 if (likely(!cpu_hotplug.refcount))
>                         break;
>                 __set_current_state(TASK_UNINTERRUPTIBLE);
> --
> 1.8.1.5
>



-- 
Pranith
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