Re: [PATCH v4] lib/spinlock_debug.c: prevent a recursive cycle in the debug code

Byungchul Park
2016-01-28T01:50:09Z

+cc peter@hurleysoftware.com
+cc sergey.senozhatsky.work@gmail.com

On Wed, Jan 27, 2016 at 09:01:01PM +0900, Byungchul Park wrote:
> changes form v3 to v4
> - reuse a existing code as much as possible for preventing an infinite
> recursive cycle.
>
> changes from v2 to v3
> - avoid printk() only in case of lockup suspected, not real lockup in
> which case it does not help at all.
> - consider not only console_sem.lock but also logbuf_lock which is used
> by printk().
>
> changes from v1 to v2
> - only change comment and commit message esp. replacing "deadlock" with
> "infinite recursive cycle", since it is not an actual deadlock.
>
> thanks,
> byungchul
>
> -----8<-----
> >From 7b0c6e48625632fa1732b619083dc550b5d924c6 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul.park@lge.com>
> Date: Wed, 27 Jan 2016 18:11:55 +0900
> Subject: [PATCH v4] lib/spinlock_debug.c: prevent a recursive cycle in the
> debug code
>
> It causes an infinite recursive cycle when using CONFIG_DEBUG_SPINLOCK,
> in the spin_dump(). Backtrace prints printk() -> console_trylock() ->
> do_raw_spin_lock() -> spin_dump() -> printk()... infinitely.
>
> When the debug spinlock code is called from printk(), we should prevent
> calling spin_dump() and the like, calling printk() again in that context.
>
> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> ---
> include/linux/debug_locks.h | 4 ++++
> kernel/locking/spinlock_debug.c | 14 +++++++++++---
> 2 files changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/debug_locks.h b/include/linux/debug_locks.h
> index 822c135..b0f977e 100644
> --- a/include/linux/debug_locks.h
> +++ b/include/linux/debug_locks.h
> @@ -10,6 +10,10 @@ struct task_struct;
> extern int debug_locks;
> extern int debug_locks_silent;
>
> +static inline void __debug_locks_on(void)
> +{
> + debug_locks = 1;
> +}
>
> static inline int __debug_locks_off(void)
> {
> diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
> index 0374a59..65177ba 100644
> --- a/kernel/locking/spinlock_debug.c
> +++ b/kernel/locking/spinlock_debug.c
> @@ -113,11 +113,19 @@ static void __spin_lock_debug(raw_spinlock_t *lock)
> return;
> __delay(1);
> }
> - /* lockup suspected: */
> - spin_dump(lock, "lockup suspected");
> +
> + /*
> + * We should prevent calling printk() further, since it would cause
> + * an infinite recursive cycle if it's called from printk()!
> + */
> + if (__debug_locks_off()) {
> + /* lockup suspected: */
> + spin_dump(lock, "lockup suspected");
> #ifdef CONFIG_SMP
> - trigger_all_cpu_backtrace();
> + trigger_all_cpu_backtrace();
> #endif
> + __debug_locks_on();
> + }
>
> /*
> * The trylock above was causing a livelock. Give the lower level arch
> --
> 1.9.1