From 0ba4f4bb1aaf7e36cd8efb820691c9bceb487859 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Fri, 15 Jul 2016 15:08:20 -0400 Subject: [PATCH] printk: do cond_resched() between lines while outputting to consoles commit 8d91f8b15361dfb438ab6eb3b319e2ded43458ff upstream. @console_may_schedule tracks whether console_sem was acquired through lock or trylock. If the former, we're inside a sleepable context and console_conditional_schedule() performs cond_resched(). This allows console drivers which use console_lock for synchronization to yield while performing time-consuming operations such as scrolling. However, the actual console outputting is performed while holding irq-safe logbuf_lock, so console_unlock() clears @console_may_schedule before starting outputting lines. Also, only a few drivers call console_conditional_schedule() to begin with. This means that when a lot of lines need to be output by console_unlock(), for example on a console registration, the task doing console_unlock() may not yield for a long time on a non-preemptible kernel. If this happens with a slow console devices, for example a serial console, the outputting task may occupy the cpu for a very long time. Long enough to trigger softlockup and/or RCU stall warnings, which in turn pile more messages, sometimes enough to trigger the next cycle of warnings incapacitating the system. Fix it by making console_unlock() insert cond_resched() between lines if @console_may_schedule. Signed-off-by: Tejun Heo Reported-by: Calvin Owens Acked-by: Jan Kara Cc: Dave Jones Cc: Kyle McMartin Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds [ciwillia@brocade.com: adjust context for 3.10.y] Signed-off-by: Chas Williams Signed-off-by: Willy Tarreau --- include/linux/console.h | 1 + kernel/panic.c | 3 +++ kernel/printk.c | 35 ++++++++++++++++++++++++++++++++++- 3 files changed, 38 insertions(+), 1 deletion(-) diff --git a/include/linux/console.h b/include/linux/console.h index 73bab0f58af5..6877ffc97d8c 100644 --- a/include/linux/console.h +++ b/include/linux/console.h @@ -153,6 +153,7 @@ extern int console_trylock(void); extern void console_unlock(void); extern void console_conditional_schedule(void); extern void console_unblank(void); +extern void console_flush_on_panic(void); extern struct tty_driver *console_device(int *); extern void console_stop(struct console *); extern void console_start(struct console *); diff --git a/kernel/panic.c b/kernel/panic.c index 167ec097ce8b..d3d74c4e2258 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -22,6 +22,7 @@ #include #include #include +#include #define PANIC_TIMER_STEP 100 #define PANIC_BLINK_SPD 18 @@ -128,6 +129,8 @@ void panic(const char *fmt, ...) bust_spinlocks(0); + console_flush_on_panic(); + if (!panic_blink) panic_blink = no_blink; diff --git a/kernel/printk.c b/kernel/printk.c index fd0154a57d6e..ee8f6be7d8a9 100644 --- a/kernel/printk.c +++ b/kernel/printk.c @@ -2033,13 +2033,24 @@ void console_unlock(void) static u64 seen_seq; unsigned long flags; bool wake_klogd = false; - bool retry; + bool do_cond_resched, retry; if (console_suspended) { up(&console_sem); return; } + /* + * Console drivers are called under logbuf_lock, so + * @console_may_schedule should be cleared before; however, we may + * end up dumping a lot of lines, for example, if called from + * console registration path, and should invoke cond_resched() + * between lines if allowable. Not doing so can cause a very long + * scheduling stall on a slow console leading to RCU stall and + * softlockup warnings which exacerbate the issue with more + * messages practically incapacitating the system. + */ + do_cond_resched = console_may_schedule; console_may_schedule = 0; /* flush buffered message fragment immediately to console */ @@ -2096,6 +2107,9 @@ skip: call_console_drivers(level, text, len); start_critical_timings(); local_irq_restore(flags); + + if (do_cond_resched) + cond_resched(); } console_locked = 0; mutex_release(&console_lock_dep_map, 1, _RET_IP_); @@ -2164,6 +2178,25 @@ void console_unblank(void) console_unlock(); } +/** + * console_flush_on_panic - flush console content on panic + * + * Immediately output all pending messages no matter what. + */ +void console_flush_on_panic(void) +{ + /* + * If someone else is holding the console lock, trylock will fail + * and may_schedule may be set. Ignore and proceed to unlock so + * that messages are flushed out. As this can be called from any + * context and we don't want to get preempted while flushing, + * ensure may_schedule is cleared. + */ + console_trylock(); + console_may_schedule = 0; + console_unlock(); +} + /* * Return the console tty driver structure and its associated index */ -- 2.20.1