Re: [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t

[Sergey Senozhatsky]

On (20/12/09 18:22), Sergey Senozhatsky wrote:
> > 
> > Please put on your eye cancer gear and inspect the atomic implementation
> > of PA-RISC, Sparc32, feh, I forgot who else.
> > 
> > Those SMP capable architectures are gifted with just one XCHG like
> > atomic instruction :/ Anyway, as said in the other email, they also
> > don't have NMIs so it mostly works.

PeterZ, thanks for the pointers!


> Hmm, wow. OK, I definitely want to look further.
> 
> When some CONFIG_DEBUG_FOO_BAR code wants to pr_err from prb->atomic_op
> on those archs then we deadlock in printk once again?

E.g. arch/sparc/lib/atomic32.c

	spinlock_t __atomic_hash[ATOMIC_HASH_SIZE];
	atomic_foo()
	{
		spin_lock_irqsave(ATOMIC_HASH(v), flags)
		...
		spin_unlock_irqrestore(ATOMIC_HASH(v), flags);
	}

So another potential re-entry path is

	atomic_foo()
	 spin_lock_irqsave(ATOMIC_HASH(v), flags)
	  printk()
	   prb()
	    atomic_foo()
	     spin_lock_irqsave(ATOMIC_HASH(v), flags)

which can deadlock, in theory, if both atomics HASH to the same
key (same spin_lock).

I wonder what else am I missing.

	-ss