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

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Date:	Sat, 23 Jan 2016 17:15:09 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...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@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: [RFC][PATCH v3 0/4] let printk()/console_trylock() callers to cond_resched()

[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').

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