linux-kernel - Re: [RFC PATCH] sysrq: don't hold the sysrq_key_table

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Date:	Mon, 26 Jul 2010 16:34:20 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	Xiaotian Feng <dfeng@...hat.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC PATCH] sysrq: don't hold the sysrq_key_table_lock during
 the handler

On Mon, Jul 26, 2010 at 10:41:54AM -0700, Dmitry Torokhov wrote:
> On Mon, Jul 26, 2010 at 06:51:48AM -0400, Neil Horman wrote:
> > On Mon, Jul 26, 2010 at 05:54:02PM +0800, Xiaotian Feng wrote:
> > > sysrq_key_table_lock is used to protect the sysrq_key_table, make sure
> > > we get/replace the right operation for the sysrq. But in __handle_sysrq,
> > > kernel will hold this lock and disable irqs until we finished op_p->handler().
> > > This may cause false positive watchdog alert when we're doing "show-task-states"
> > > on a system with many tasks.
> > > 
> > > Signed-off-by: Xiaotian Feng <dfeng@...hat.com>
> > > Cc: Ingo Molnar <mingo@...e.hu>
> > > Cc: Dmitry Torokhov <dmitry.torokhov@...il.com>
> > > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > > Cc: Neil Horman <nhorman@...driver.com>
> > > Cc: "David S. Miller" <davem@...emloft.net>
> > > ---
> > >  drivers/char/sysrq.c |    4 +++-
> > >  1 files changed, 3 insertions(+), 1 deletions(-)
> > > 
> > > diff --git a/drivers/char/sysrq.c b/drivers/char/sysrq.c
> > > index 878ac0c..0856e2e 100644
> > > --- a/drivers/char/sysrq.c
> > > +++ b/drivers/char/sysrq.c
> > > @@ -520,9 +520,11 @@ void __handle_sysrq(int key, struct tty_struct *tty, int check_mask)
> > >  		if (!check_mask || sysrq_on_mask(op_p->enable_mask)) {
> > >  			printk("%s\n", op_p->action_msg);
> > >  			console_loglevel = orig_log_level;
> > > +			spin_unlock_irqrestore(&sysrq_key_table_lock, flags);
> > >  			op_p->handler(key, tty);
> > >  		} else {
> > >  			printk("This sysrq operation is disabled.\n");
> > > +			spin_unlock_irqrestore(&sysrq_key_table_lock, flags);
> > >  		}
> > >  	} else {
> > >  		printk("HELP : ");
> > > @@ -541,8 +543,8 @@ void __handle_sysrq(int key, struct tty_struct *tty, int check_mask)
> > >  		}
> > >  		printk("\n");
> > >  		console_loglevel = orig_log_level;
> > > +		spin_unlock_irqrestore(&sysrq_key_table_lock, flags);
> > >  	}
> > > -	spin_unlock_irqrestore(&sysrq_key_table_lock, flags);
> > >  }
> > >  
> > >  void handle_sysrq(int key, struct tty_struct *tty)
> > > -- 
> > > 1.7.2
> > > 
> > > 
> > 
> > This creates the possibility of a race in the handler.  Not that it happens
> > often, but sysrq keys can be registered and unregistered dynamically.  If that
> > lock isn't held while we call the keys handler, the code implementing that
> > handler can live in a module that gets removed while its executing, leading to
> > an oops, etc.  I think the better solution would be to use an rcu lock here.
> 
> I'd simply changed spinlock to a mutex.
> 
I don't think you can do that safely in this path, as sysrqs will be looked up
in both process (echo t > /proc/sysrq-trigger) context and in interrupt
(alt-sysrq-t) context.  If a mutex is locked and you try to take it in interrupt
context, you get a sleeping-in-interrupt panic IIRC

Neil

> -- 
> Dmitry
> 
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