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    SubjectRe: [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@redhat.com>
    > > > Cc: Ingo Molnar <mingo@elte.hu>
    > > > Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
    > > > Cc: Andrew Morton <akpm@linux-foundation.org>
    > > > Cc: Neil Horman <nhorman@tuxdriver.com>
    > > > Cc: "David S. Miller" <davem@davemloft.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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