Re: [PATCH] printk: Fix spinlock deadlock in printk reenty

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 30 Nov 2016 15:15:19 +0800 <linyongting@huawei.com> wrote:

> From: Jinling Ke <kejinling@huawei.com>
> 
> when Oops in printk, printk will call zap_locks() to reinitialize
> spinlock to prevent deadlock. In arm, arm64, x86 or other
> architecture smp cpu, race condition will occur in printk spinlock
> logbuf_lock and then it will result other cpu that is waiting printk
> spinlock in deadlock(in function raw_spin_lock). Because the cpus
> deadlock, you can see the error  printk log:
> 
> "SMP: failed to stop secondary CPUs"
> 
> In arm, arm64, x86 or other architecture, spinlock variable
> is divided into 2 parts, for example they are 'owner' and 'next' in arm.
> When get a spinlock, the 'next' part will add 1 and wait 'next' being
> equal to 'owner'. However, at this moment, the 'next' part is local
> variable, but 'owner' part value is get from global variable logbuf_lock.
> However,raw_spin_lock_init(&logbuf_lock) will set 'owner' part and
> 'next' part to zero, the result is that cpu deadlock in function
> raw_spin_lock( while loop in function arch_spin_lock ).
> 
> 	struct of arm spinlock
> 	 	union {
> 			u32 slock;
> 			struct __raw_tickets {
> 				u16 owner;
> 				u16 next;
> 			} tickets;
> 		};
> 	} arch_spinlock_t;
> 	static inline void arch_spin_lock(arch_spinlock_t *lock)
> 	{...
> 		<--- At the moment, other cpu call zap_locks()->spin_lock_init(),
> 		<--- set the 'owner' part to zero, but lockval.tickets.next is a
> 	        <--- local variable
> 		while (lockval.tickets.next != lockval.tickets.owner) {
> 			lockval.tickets.owner = ACCESS_ONCE(lock->tickets.owner);
> 		}
> 	...
> 	}
> 
> The solution is that In function zap_locks(), replace
> raw_spin_lock_init(&logbuf_lock) with raw_spin_unlock(&logbuf_lock),
> to let spin_lock stay in unlocked.
> 
> ...
>
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1603,7 +1603,7 @@ static void zap_locks(void)
>  
>  	debug_locks_off();
>  	/* If a crash is occurring, make sure we can't deadlock */
> -	raw_spin_lock_init(&logbuf_lock);
> +	raw_spin_unlock(&logbuf_lock);
>  	/* And make sure that we print immediately */
>  	sema_init(&console_sem, 1);

OK, so it's a race between raw_spin_lock() and raw_spin_lock_init()?

I wonder if there's a more general way of preventing this, within
raw_spin_lock_init()?

Of course, printk is special and the situation is unlikely to occur
elsewhere.

I guess the raw_spin_unlock() is OK - lockdep would have warned about
unlock-of-unlocked-lock but we did a debug_locks_off() to prevent that.