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From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	John Ogness <john.ogness@linutronix.de>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Petr Mladek <pmladek@suse.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t
Date: Wed, 9 Dec 2020 19:46:13 +0900	[thread overview]
Message-ID: <X9Cq9SMaCy3lGhXJ@jagdpanzerIV.localdomain> (raw)
In-Reply-To: <20201209092204.GD1667627@google.com>

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

  reply	other threads:[~2020-12-09 10:46 UTC|newest]

Thread overview: 47+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-01 20:53 [PATCH next v2 0/3] printk: remove logbuf_lock John Ogness
2020-12-01 20:53 ` [PATCH next v2 1/3] printk: inline log_output(),log_store() in vprintk_store() John Ogness
2020-12-03 15:57   ` Petr Mladek
2020-12-03 16:25     ` John Ogness
2020-12-04  6:13       ` Sergey Senozhatsky
2020-12-04  8:26       ` Petr Mladek
2020-12-01 20:53 ` [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t John Ogness
2020-12-04  9:12   ` Petr Mladek
2020-12-06 20:23     ` John Ogness
2020-12-07  9:34       ` Peter Zijlstra
2020-12-07 10:03         ` John Ogness
2020-12-07 12:56           ` Peter Zijlstra
2020-12-07 12:56           ` Petr Mladek
2020-12-07 16:46           ` David Laight
2020-12-08 20:34     ` Sergey Senozhatsky
2020-12-08 22:30       ` John Ogness
2020-12-09  1:04         ` Sergey Senozhatsky
2020-12-09  8:16         ` Peter Zijlstra
2020-12-09  9:22           ` Sergey Senozhatsky
2020-12-09 10:46             ` Sergey Senozhatsky [this message]
2020-12-09 11:00               ` Peter Zijlstra
2020-12-09 11:28                 ` Sergey Senozhatsky
2020-12-09 12:29                   ` Peter Zijlstra
2020-12-09  8:07       ` Peter Zijlstra
2020-12-01 20:53 ` [PATCH next v2 3/3] printk: remove logbuf_lock, add syslog_lock John Ogness
2020-12-04  6:41   ` Sergey Senozhatsky
2020-12-06 20:44     ` John Ogness
2020-12-04 15:52   ` devkmsg: was " Petr Mladek
2020-12-06 20:51     ` John Ogness
2020-12-07  9:56       ` Petr Mladek
2020-12-04 15:57   ` syslog: was: " Petr Mladek
2020-12-06 21:06     ` John Ogness
2020-12-07 10:01       ` Petr Mladek
2020-12-04 16:10   ` recursion handling: " Petr Mladek
2020-12-05  4:25     ` Sergey Senozhatsky
2020-12-06 22:08       ` John Ogness
2020-12-05  9:41     ` Sergey Senozhatsky
2020-12-06 22:17       ` John Ogness
2020-12-06 21:44     ` John Ogness
2020-12-07 11:17       ` Petr Mladek
2020-12-04 16:15   ` vprintk_store: was: " Petr Mladek
2020-12-06 22:30     ` John Ogness
2020-12-07 12:46       ` Petr Mladek
2020-12-04 16:19   ` consoles: " Petr Mladek
2020-12-05  4:39     ` Sergey Senozhatsky
2020-12-07  9:50       ` Petr Mladek
2020-12-08 20:51         ` Sergey Senozhatsky

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