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

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Date:   Wed, 9 Dec 2020 13:29:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        John Ogness <john.ogness@...utronix.de>,
        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 Wed, Dec 09, 2020 at 08:28:52PM +0900, Sergey Senozhatsky wrote:
> On (20/12/09 12:00), Peter Zijlstra wrote:
> > > 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).
> > 
> > Yep, but see the 'mostly' in the 'they mostly work'. Given the
> > limitiations of these architectures there's really only so much you can
> > do.
> 
> Right, agreed.
> 
> Nevertheless TIL that lockless printk buffer is not always lockless.
> Perhaps, people that work with those archs need to also know this.

Last time I broke them, they were aware they're 'special' and IIRC
they're mostly just limping along on prayers.

> I haven't checked all the archs, but if, somehow, (IF) some of them
> can panic the system with the atomic hash entries locked, then on
> those archs new printk may not be able to flush-on-panic. Because
> while printk iterates logbuf it may HASH to the atomic hash table
> entry, that will never be unlocked. So there are some changes in
> atomic/printk department on those archs.

Yeah, so I wouldn't put too much effort into thinking about it.
Hopefully we can eventually delete these architectures and really forget
they exist.