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. v4: -- added "printk: check CON_ENABLED in have_callable_console()" patch -- drop a "optional optimization" patch from the series (Petr Mladek) -- merge 0001 and "drop console_trylock_for_printk" patches (Petr Mladek) -- tweak console_trylock() comment (Petr Mladek) 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 ([email protected]) -- take care of RCU preempt kernels in console_trylock() Sergey Senozhatsky (3): printk: move can_use_console out of console_trylock_for_printk printk: set may_schedule for some of console_trylock callers printk: check CON_ENABLED in have_callable_console() kernel/printk/printk.c | 121 +++++++++++++++++++++++-------------------------- 1 file changed, 57 insertions(+), 64 deletions(-) -- 2.7.1
