Re: [RFC PATCH] sysrq: don't hold the sysrq_key_table_lock duringthe handler

[Dmitry Torokhov]

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@xxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxx>
> > Cc: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Neil Horman <nhorman@xxxxxxxxxxxxx>
> > Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> > ---
> >  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.

-- 
Dmitry
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