From 81cc26f2bd11ba4421a17a2d5cebe4bba206c239 Mon Sep 17 00:00:00 2001 From: Thierry Reding Date: Fri, 15 Jan 2016 16:58:21 -0800 Subject: printk: only unregister boot consoles when necessary Boot consoles are typically replaced by proper consoles during the boot process. This can be problematic if the boot console data is part of the init section that is reclaimed late during boot. If the proper console does not register before this point in time, the boot console will need to be removed (so that the freed memory is not accessed), leaving the system without output for some time. There are various reasons why the proper console may not register early enough, such as deferred probe or the driver being a loadable module. If that happens, there is some amount of time where no console messages are visible to the user, which in turn can mean that they won't see crashes or other potentially useful information. To avoid this situation, only remove the boot console when it resides in the init section. Code exists to replace the boot console by the proper console when it is registered, keeping a seamless transition between the boot and proper consoles. Signed-off-by: Thierry Reding Cc: Arnd Bergmann Cc: Greg Kroah-Hartman Cc: Joe Perches Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/printk/printk.c | 26 +++++++++++++++++++++++++- 1 file changed, 25 insertions(+), 1 deletion(-) (limited to 'kernel/printk/printk.c') diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 2ce8826f1053..0c9f02506169 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -48,6 +48,7 @@ #include #include +#include #define CREATE_TRACE_POINTS #include @@ -2658,13 +2659,36 @@ int unregister_console(struct console *console) } EXPORT_SYMBOL(unregister_console); +/* + * Some boot consoles access data that is in the init section and which will + * be discarded after the initcalls have been run. To make sure that no code + * will access this data, unregister the boot consoles in a late initcall. + * + * If for some reason, such as deferred probe or the driver being a loadable + * module, the real console hasn't registered yet at this point, there will + * be a brief interval in which no messages are logged to the console, which + * makes it difficult to diagnose problems that occur during this time. + * + * To mitigate this problem somewhat, only unregister consoles whose memory + * intersects with the init section. Note that code exists elsewhere to get + * rid of the boot console as soon as the proper console shows up, so there + * won't be side-effects from postponing the removal. + */ static int __init printk_late_init(void) { struct console *con; for_each_console(con) { if (!keep_bootcon && con->flags & CON_BOOT) { - unregister_console(con); + /* + * Make sure to unregister boot consoles whose data + * resides in the init section before the init section + * is discarded. Boot consoles whose data will stick + * around will automatically be unregistered when the + * proper console replaces them. + */ + if (init_section_intersects(con, sizeof(*con))) + unregister_console(con); } } hotcpu_notifier(console_cpu_notify, 0); -- cgit v1.2.3-59-g8ed1b From 8d91f8b15361dfb438ab6eb3b319e2ded43458ff Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Fri, 15 Jan 2016 16:58:24 -0800 Subject: printk: do cond_resched() between lines while outputting to consoles @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 --- include/linux/console.h | 1 + kernel/panic.c | 3 +-- kernel/printk/printk.c | 35 ++++++++++++++++++++++++++++++++++- 3 files changed, 36 insertions(+), 3 deletions(-) (limited to 'kernel/printk/printk.c') diff --git a/include/linux/console.h b/include/linux/console.h index bd194343c346..ea731af2451e 100644 --- a/include/linux/console.h +++ b/include/linux/console.h @@ -150,6 +150,7 @@ 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 b333380c6bb2..d96469de72dc 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -180,8 +180,7 @@ void panic(const char *fmt, ...) * panic() is not being callled from OOPS. */ debug_locks_off(); - console_trylock(); - console_unlock(); + console_flush_on_panic(); if (!panic_blink) panic_blink = no_blink; diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 0c9f02506169..08934a395c1a 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2234,13 +2234,24 @@ 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 */ @@ -2312,6 +2323,9 @@ skip: call_console_drivers(level, ext_text, ext_len, text, len); start_critical_timings(); local_irq_restore(flags); + + if (do_cond_resched) + cond_resched(); } console_locked = 0; @@ -2379,6 +2393,25 @@ 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 */ -- cgit v1.2.3-59-g8ed1b From 06b031de22d28ae76b2e5bfaf22c56a265a1e106 Mon Sep 17 00:00:00 2001 From: Sergey Senozhatsky Date: Fri, 15 Jan 2016 16:59:23 -0800 Subject: printk: change recursion_bug type to bool `recursion_bug' is used as recursion_bug toggle, so make it `bool'. Signed-off-by: Sergey Senozhatsky Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/printk/printk.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) (limited to 'kernel/printk/printk.c') diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 08934a395c1a..e79439134978 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1661,7 +1661,7 @@ asmlinkage int vprintk_emit(int facility, int level, const char *dict, size_t dictlen, const char *fmt, va_list args) { - static int recursion_bug; + static bool recursion_bug; static char textbuf[LOG_LINE_MAX]; char *text = textbuf; size_t text_len = 0; @@ -1697,7 +1697,7 @@ asmlinkage int vprintk_emit(int facility, int level, * it can be printed at the next appropriate moment: */ if (!oops_in_progress && !lockdep_recursing(current)) { - recursion_bug = 1; + recursion_bug = true; local_irq_restore(flags); return 0; } @@ -1712,7 +1712,7 @@ asmlinkage int vprintk_emit(int facility, int level, static const char recursion_msg[] = "BUG: recent printk recursion!"; - recursion_bug = 0; + recursion_bug = false; /* emit KERN_CRIT message */ printed_len += log_store(0, 2, LOG_PREFIX|LOG_NEWLINE, 0, NULL, 0, recursion_msg, -- cgit v1.2.3-59-g8ed1b