Re: [PATCHv6 6/7] printk: use printk_safe buffers in printk

[Sergey Senozhatsky <sergey.senozhatsky.work@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