linux-kernel - Re: [PATCH next v2 2/3] printk: change @clear_seq to atomic64

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Date:   Wed, 9 Dec 2020 19:46:13 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t

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