Re: [PATCH] lock/semaphore: Avoid a deadlock within __up()

[Ingo Molnar]

* Byungchul Park <byungchul.park@xxxxxxx> wrote:

> Since I faced a infinite recursive printk() bug, I've tried to propose
> patches the title of which is "lib/spinlock_debug.c: prevent a recursive
> cycle in the debug code". But I noticed the root problem cannot be fixed
> by that, through some discussion thanks to Sergey and Peter. So I focused
> on preventing the DEADLOCK.
> 
> -----8<-----
> From 94a66990677735459a7790b637179d8600479639 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul.park@xxxxxxx>
> Date: Tue, 2 Feb 2016 15:35:48 +0900
> Subject: [PATCH] lock/semaphore: Avoid a deadlock within __up()
> 
> When the semaphore __up() is called from within printk() with
> console_sem.lock, a DEADLOCK can happen, since the wake_up_process() can
> call printk() again, esp. if defined CONFIG_DEBUG_SPINLOCK. And the
> wake_up_process() don't need to be within a critical section.
> 
> The scenario the bad thing can happen is,
> 
> printk
>   console_trylock
>   console_unlock
>     up_console_sem
>       up
>         raw_spin_lock_irqsave(&sem->lock, flags)
>         __up
>           wake_up_process
>             try_to_wake_up
>               raw_spin_lock_irqsave(&p->pi_lock)
>                 __spin_lock_debug
>                   spin_dump
>                     printk
>                       console_trylock
>                         raw_spin_lock_irqsave(&sem->lock, flags)
> 
>                         *** DEADLOCK ***
> 
> Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
> ---
>  kernel/locking/semaphore.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
> index b8120ab..d3a28dc 100644
> --- a/kernel/locking/semaphore.c
> +++ b/kernel/locking/semaphore.c
> @@ -259,5 +259,14 @@ static noinline void __sched __up(struct semaphore *sem)
>  						struct semaphore_waiter, list);
>  	list_del(&waiter->list);
>  	waiter->up = true;
> +
> +	/*
> +	 * Trying to acquire this sem->lock in wake_up_process() leads a
> +	 * DEADLOCK unless we unlock it here. For example, it's possile
> +	 * in the case that called from within printk() since
> +	 * wake_up_process() might call printk().
> +	 */
> +	raw_spin_unlock_irq(&sem->lock);
>  	wake_up_process(waiter->task);
> +	raw_spin_lock_irq(&sem->lock);

So I'm pretty sad about this solution, as it penalizes every semaphore user - 
while the deadlock is a really obscure one occuring within the scheduler or a 
console driver, which are very narrow code paths!

(Also, please don't shout in comments, unless there's some really good reason to 
do it.)

Why doesn't spin_dump() break the console lock instead, if it detects that it's 
spinning on it, before doing the printk()? It's a likely fail state anyway - and 
this way we push any intrusive debug oriented action towards the unlikely fail 
state.

Alternatively: why not improve down_trylock() to be lockless? The main reason for 
the lockup is that a trylock op takes the semaphore spinlock unconditionally. 
Which is fine for legacy code, but could perhaps be improved upon - I think we 
could in fact do it without turning sem->count into atomics.

Alternatively #2: move printk() away from semaphores - it's pretty special code 
anyway and semaphore semanthics are far from obvious.

Thanks,

	Ingo