path: root/kernel/panic.c
diff options
authorSergey Senozhatsky <sergey.senozhatsky.work@gmail.com>2018-10-25 19:10:36 +0900
committerPetr Mladek <pmladek@suse.com>2018-11-22 14:07:43 +0100
commitc7c3f05e341a9a2bd1a92993d4f996cfd6e7348e (patch)
tree2f3324a03277773a4aaf72c05162fbc6d0dbfccb /kernel/panic.c
parenta67eefad996f007afc7c38efe8f45257fb1e3994 (diff)
panic: avoid deadlocks in re-entrant console drivers
From printk()/serial console point of view panic() is special, because it may force CPU to re-enter printk() or/and serial console driver. Therefore, some of serial consoles drivers are re-entrant. E.g. 8250: serial8250_console_write() { if (port->sysrq) locked = 0; else if (oops_in_progress) locked = spin_trylock_irqsave(&port->lock, flags); else spin_lock_irqsave(&port->lock, flags); ... } panic() does set oops_in_progress via bust_spinlocks(1), so in theory we should be able to re-enter serial console driver from panic(): CPU0 <NMI> uart_console_write() serial8250_console_write() // if (oops_in_progress) // spin_trylock_irqsave() call_console_drivers() console_unlock() console_flush_on_panic() bust_spinlocks(1) // oops_in_progress++ panic() <NMI/> spin_lock_irqsave(&port->lock, flags) // spin_lock_irqsave() serial8250_console_write() call_console_drivers() console_unlock() printk() ... However, this does not happen and we deadlock in serial console on port->lock spinlock. And the problem is that console_flush_on_panic() called after bust_spinlocks(0): void panic(const char *fmt, ...) { bust_spinlocks(1); ... bust_spinlocks(0); console_flush_on_panic(); ... } bust_spinlocks(0) decrements oops_in_progress, so oops_in_progress can go back to zero. Thus even re-entrant console drivers will simply spin on port->lock spinlock. Given that port->lock may already be locked either by a stopped CPU, or by the very same CPU we execute panic() on (for instance, NMI panic() on printing CPU) the system deadlocks and does not reboot. Fix this by removing bust_spinlocks(0), so oops_in_progress is always set in panic() now and, thus, re-entrant console drivers will trylock the port->lock instead of spinning on it forever, when we call them from console_flush_on_panic(). Link: http://lkml.kernel.org/r/20181025101036.6823-1-sergey.senozhatsky@gmail.com Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Daniel Wang <wonderfly@google.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk> Cc: Jiri Slaby <jslaby@suse.com> Cc: Peter Feiner <pfeiner@google.com> Cc: linux-serial@vger.kernel.org Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> Cc: stable@vger.kernel.org Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Signed-off-by: Petr Mladek <pmladek@suse.com>
Diffstat (limited to 'kernel/panic.c')
1 files changed, 5 insertions, 1 deletions
diff --git a/kernel/panic.c b/kernel/panic.c
index 8b2e002d52eb..6a6df23acd1a 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -14,6 +14,7 @@
#include <linux/kmsg_dump.h>
#include <linux/kallsyms.h>
#include <linux/notifier.h>
+#include <linux/vt_kern.h>
#include <linux/module.h>
#include <linux/random.h>
#include <linux/ftrace.h>
@@ -233,7 +234,10 @@ void panic(const char *fmt, ...)
if (_crash_kexec_post_notifiers)
- bust_spinlocks(0);
+#ifdef CONFIG_VT
+ unblank_screen();
+ console_unblank();
* We may have ended up stopping the CPU holding the lock (in

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