From 6b97a20d3a7909daa06625d4440c2c52d7bf08d7 Mon Sep 17 00:00:00 2001 From: Sergey Senozhatsky Date: Thu, 17 Mar 2016 14:21:23 -0700 Subject: [PATCH] printk: set may_schedule for some of console_trylock() callers console_unlock() allows to cond_resched() if its caller has set `console_may_schedule' to 1, since 8d91f8b15361 ("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 However, console_trylock() callers (among them is printk()) do not always call printk() from atomic contexts, and some of them can cond_resched() in console_unlock(), so console_trylock() can set `console_may_schedule' to 1 for such processes. For !CONFIG_PREEMPT_COUNT kernels, however, console_trylock() always sets `console_may_schedule' to 0. It's possible to drop explicit preempt_disable()/preempt_enable() in vprintk_emit(), because console_unlock() and console_trylock() are now smart enough: a) console_unlock() does not cond_resched() when it's unsafe (console_trylock() takes care of that) b) console_unlock() does can_use_console() check. Signed-off-by: Sergey Senozhatsky Reviewed-by: Petr Mladek Cc: Jan Kara Cc: Tejun Heo Cc: Kyle McMartin Cc: Dave Jones Cc: Calvin Owens Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/printk/printk.c | 23 ++++++++++++++--------- 1 file changed, 14 insertions(+), 9 deletions(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 2523332bd998..a6d023c3b852 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1757,13 +1757,6 @@ asmlinkage int vprintk_emit(int facility, int level, /* If called from the scheduler, we can not call up(). */ if (!in_sched) { lockdep_off(); - /* - * Disable preemption to avoid being preempted while holding - * console_sem which would prevent anyone from printing to - * console - */ - preempt_disable(); - /* * Try to acquire and then immediately release the console * semaphore. The release will print out buffers and wake up @@ -1771,7 +1764,6 @@ asmlinkage int vprintk_emit(int facility, int level, */ if (console_trylock()) console_unlock(); - preempt_enable(); lockdep_on(); } @@ -2122,7 +2114,20 @@ int console_trylock(void) return 0; } console_locked = 1; - console_may_schedule = 0; + /* + * When PREEMPT_COUNT disabled we can't reliably detect if it's + * safe to schedule (e.g. calling printk while holding a spin_lock), + * because preempt_disable()/preempt_enable() are just barriers there + * and preempt_count() is always 0. + * + * RCU read sections have a separate preemption counter when + * PREEMPT_RCU enabled thus we must take extra care and check + * rcu_preempt_depth(), otherwise RCU read sections modify + * preempt_count(). + */ + console_may_schedule = !oops_in_progress && + preemptible() && + !rcu_preempt_depth(); return 1; } EXPORT_SYMBOL(console_trylock);