[RFC][PATCH v3 0/4] let printk()/console_trylock() callers to cond_resched()

From: Sergey Senozhatsky
Date: Sat Jan 23 2016 - 03:18:05 EST

[was: cond_resched() some of console_trylock callers]


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)

-- make have_callable_console() available for !PRINTK configs (lkp@xxxxxxxxx)
-- 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(-)