3.16.65-rc1 review patch. If anyone has any objections, please let me know.
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From: Sergey Senozhatsky sergey.senozhatsky.work@gmail.com
commit c7c3f05e341a9a2bd1a92993d4f996cfd6e7348e upstream.
=46romprintk()/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 Signed-off-by: Sergey Senozhatsky sergey.senozhatsky@gmail.com Signed-off-by: Petr Mladek pmladek@suse.com Signed-off-by: Ben Hutchings ben@decadent.org.uk --- kernel/panic.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)
--- a/kernel/panic.c +++ b/kernel/panic.c @@ -13,6 +13,7 @@ #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> @@ -178,7 +179,10 @@ void panic(const char *fmt, ...) */ crash_kexec(NULL);
- bust_spinlocks(0); +#ifdef CONFIG_VT + unblank_screen(); +#endif + console_unblank();
/* * We may have ended up stopping the CPU holding the lock (in