linux-kernel - Re: [RFC][PATCH v3 0/4] let printk()/console_trylock() callers to cond

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Message-ID: <20160203034930.GA718@swordfish>
Date:	Wed, 3 Feb 2016 12:49:30 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Petr Mladek <pmladek@...e.com>, Jan Kara <jack@...e.com>,
	Tejun Heo <tj@...nel.org>, Kyle McMartin <kyle@...nel.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Calvin Owens <calvinowens@...com>,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC][PATCH v3 0/4] let printk()/console_trylock() callers to
 cond_resched()

Hello,

On (01/23/16 17:15), Sergey Senozhatsky wrote:
> [was: cond_resched() some of console_trylock callers]
> 
> Hello,
> 
> console_unlock() allows to cond_resched() if its caller has
> set `console_may_schedule' to 1 (this functionality present
> since commit 'printk: do cond_resched() between lines while
> outputting to consoles').

gentlemen, did you get a chance to look into this?

	-ss

> The rules are:
> -- console_lock() always sets `console_may_schedule' to 1
> -- console_trylock() always sets `console_may_schedule' to 0
> 
> printk() calls console_unlock() with preemption desabled, which
> basically can lead to RCU stalls, watchdog soft lockups, etc. if
> something is simultaneously calling printk() frequent enough (IOW,
> console_sem owner always has new data to send to console divers
> and can't leave console_unlock() for a long time).
> 
> printk()->console_trylock() callers do not necessarily execute in
> atomic contexts, and some of them can cond_resched() in
> console_unlock(). console_trylock() can set `console_may_schedule'
> to 1 (allow cond_resched() later in consoe_unlock()) when it's safe.
> 
> 0001 addresses a theoretical loss of printk messages.
> 0002 is an optional optimization patch
> 0003 drops console_trylock_for_printk
> 0004 lets some of printk()/console_trylock() callers to cond_resched()
>      in console_unlock()
> 
> v2-v3 (thanks to Petr Mladek for reviews):
> -- do not call can_use_console() on every iteration in console_unlock() (Petr Mladek)
> -- move "This stops the holder of console_sem.." comment (noted by Petr Mladek)
> -- take extra care of !PREEMPT_COUNT kernels (Petr Mladek)
> -- call_console_drivers() still must check cpu_online && CON_ANYTIME (Petr Mladek)
> -- removed console_trylock_for_printk() (noted by Petr Mladek)
> 
> v1-v2:
> -- make have_callable_console() available for !PRINTK configs (lkp@...el.com)
> -- take care of RCU preempt kernels in console_trylock()
> 
> 
> Sergey Senozhatsky (4):
>   printk: move can_use_console out of console_trylock_for_printk
>   printk: do not console_cont_flush() on every jump to again
>   printk: remove console_trylock_for_printk
>   printk: set may_schedule for some of console_trylock callers
> 
>  kernel/printk/printk.c | 121 ++++++++++++++++++++++---------------------------
>  1 file changed, 55 insertions(+), 66 deletions(-)
> 
> -- 
> 2.7.0
>