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From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
	Petr Mladek <pmladek@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Jan Kara <jack@suse.cz>, Tejun Heo <tj@kernel.org>,
	Calvin Owens <calvinowens@fb.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Hurley <peter@hurleysoftware.com>,
	linux-kernel@vger.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [PATCHv6 6/7] printk: use printk_safe buffers in printk
Date: Thu, 22 Dec 2016 14:31:19 +0900	[thread overview]
Message-ID: <20161222053119.GE644@jagdpanzerIV.localdomain> (raw)
In-Reply-To: <20161221143605.2272-7-sergey.senozhatsky@gmail.com>

Hello,

On (12/21/16 23:36), Sergey Senozhatsky wrote:
> Use printk_safe per-CPU buffers in printk recursion-prone blocks:
> -- around logbuf_lock protected sections in vprintk_emit() and
>    console_unlock()
> -- around down_trylock_console_sem() and up_console_sem()
> 
> Note that this solution addresses deadlocks caused by printk()
> recursive calls only. That is vprintk_emit() and console_unlock().

several questions.

so my plan was to introduce printk-safe and to switch vprintk_emit()
and console_sem related functions (like console_unlock(), etc.) to
printk-safe first. and switch the remaining logbuf_lock users, like
devkmsg_open()/syslog_print()/etc, in a followup, pretty much
mechanical "find logbuf_lock - add printk_safe", patch. but that
followup patch is bigger than I expected (still mechanical tho);
so I want to re-group.

there are
	9 raw_spin_lock_irq(&logbuf_lock)
	7 raw_spin_lock_irqsave(&logbuf_lock, flags)
and
	12 raw_spin_lock_irq(&logbuf_lock)
	8 raw_spin_unlock_irqrestore(&logbuf_lock, flags)

wrapping each one of them in printk_safe_enter()/printk_safe_enter_irq()
and printk_safe_exit()/printk_safe_exit_irq() is a bit boring. so I have
several options: one of them is to add printk_safe_{enter,exit}_irq() and,
along with it, a bunch of help macros (to printk.c):

(questions below)

/*
 * Helper macros to lock/unlock logbuf_lock in deadlock safe
 * manner (logbuf_lock may spin_dump() in lock/unlock).
 */
#define lock_logbuf(flags)			\
	do {					\
		printk_safe_enter(flags);	\
		raw_spin_lock(&logbuf_lock);	\
	} while (0)

#define unlock_logbuf(flags)			\
	do {					\
		raw_spin_unlock(&logbuf_lock);	\
		printk_safe_exit(flags);	\
	} while (0)

#define lock_logbuf_irq()			\
	do {					\
		printk_safe_enter_irq();	\
		raw_spin_lock(&logbuf_lock);	\
	} while (0)

#define unlock_logbuf_irq()			\
	do {					\
		raw_spin_unlock(&logbuf_lock);	\
		printk_safe_exit_irq();		\
	} while (0)


so this

	printk_safe_enter_irq();
	raw_spin_lock(&logbuf_lock);
	...
	raw_spin_unlock(&logbuf_lock);
	printk_safe_exit(flags);

or this

	printk_safe_enter_irq();
	raw_spin_lock(&logbuf_lock);
	...
	raw_spin_unlock(&logbuf_lock);
	printk_safe_exit_irq();


becomes this

	lock_logbuf(flags);
	...
	unlock_logbuf(flags);

and this

	lock_logbuf_irq();
	...
	unlock_logbuf_irq();


questions:

-- the approach
 another solution? switch those raw_spin_{lock,unlock}_irq to irqsave/irqrestore
 (?) and use the existing printk_safe_enter()/printk_safe_exit(),
 so *_irq() versions of lock_logbuf/printk_safe macros will not be needed?

-- the naming
 are lock_logbuf()/unlock_logbuf() and lock_logbuf_irq()/unlock_logbuf_irq()
 good enough? (if good at all)

	-ss

  reply	other threads:[~2016-12-22  5:31 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-12-21 14:35 [PATCHv6 0/7] printk: use printk_safe to handle printk() recursive calls Sergey Senozhatsky
2016-12-21 14:35 ` [PATCHv6 1/7] printk: use vprintk_func in vprintk() Sergey Senozhatsky
2016-12-21 14:36 ` [PATCHv6 2/7] printk: rename nmi.c and exported api Sergey Senozhatsky
2016-12-21 19:45   ` Linus Torvalds
2016-12-22  1:17     ` Sergey Senozhatsky
2016-12-21 14:36 ` [PATCHv6 3/7] printk: introduce per-cpu safe_print seq buffer Sergey Senozhatsky
2016-12-22  0:53   ` kbuild test robot
2016-12-22  1:18     ` Sergey Senozhatsky
2016-12-22 16:36       ` Petr Mladek
2016-12-21 14:36 ` [PATCHv6 4/7] printk: always use deferred printk when flush printk_safe lines Sergey Senozhatsky
2016-12-21 14:36 ` [PATCHv6 5/7] printk: report lost messages in printk safe/nmi contexts Sergey Senozhatsky
2016-12-23 10:54   ` Petr Mladek
2016-12-23 15:08     ` Sergey Senozhatsky
2016-12-21 14:36 ` [PATCHv6 6/7] printk: use printk_safe buffers in printk Sergey Senozhatsky
2016-12-22  5:31   ` Sergey Senozhatsky [this message]
2016-12-22 17:10     ` Petr Mladek
2016-12-23  1:46       ` Sergey Senozhatsky
2016-12-23  9:53         ` Petr Mladek
2016-12-21 14:36 ` [PATCHv6 7/7] printk: remove zap_locks() function Sergey Senozhatsky

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