Re: [RFC][PATCHv2 1/4] panic: avoid deadlocks in re-entrant console drivers

Petr Mladek
2018-10-23T11:07:58Z

On Tue 2018-10-16 14:04:25, Sergey Senozhatsky wrote:
> >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.

The idea makes sense to me. You are right that we already called
printk/console with busted spinlock many times in panic().
Therefore it should not be worse.

> Fix this by setting oops_in_progress before console_flush_on_panic(),
> so re-entrant console drivers will trylock the port->lock instead of
> spinning on it forever.
>
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> ---
> kernel/panic.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/kernel/panic.c b/kernel/panic.c
> index f6d549a29a5c..a0e60ccf3031 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -237,7 +237,13 @@ void panic(const char *fmt, ...)
> if (_crash_kexec_post_notifiers)
> __crash_kexec(NULL);
>
> + /*
> + * Decrement oops_in_progress and let bust_spinlocks() to
> + * unblank_screen(), console_unblank() and wake_up_klogd()
> + */
> bust_spinlocks(0);
> + /* Set oops_in_progress, so we can reenter serial console driver */
> + bust_spinlocks(1);

Though this looks a bit weird.

I have just realized that console_unblank() is called by
bust_spinlocks(0) and does basically the same as
console_flush_on_panic(). Also it does not make much
sense wake_up_klogd() there. Finally, it seems to be
too late to disable lockdep there.

I would suggest something like:

diff --git a/kernel/panic.c b/kernel/panic.c
index 8b2e002d52eb..c78e3df8dd58 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -233,17 +233,14 @@ void panic(const char *fmt, ...)
if (_crash_kexec_post_notifiers)
__crash_kexec(NULL);

- bust_spinlocks(0);
-
/*
* We may have ended up stopping the CPU holding the lock (in
- * smp_send_stop()) while still having some valuable data in the console
- * buffer. Try to acquire the lock then release it regardless of the
- * result. The release will also print the buffers out. Locks debug
- * should be disabled to avoid reporting bad unlock balance when
- * panic() is not being callled from OOPS.
+ * smp_send_stop()) while still having some valuable data in
+ * the console buffer. Try hard to see them.
*/
- debug_locks_off();
+#ifdef CONFIG_VT
+ unblank_screen();
+#endif
console_flush_on_panic();

if (!panic_blink)
diff --git a/lib/bust_spinlocks.c b/lib/bust_spinlocks.c
index ab719495e2cb..e42d2fcd6453 100644
--- a/lib/bust_spinlocks.c
+++ b/lib/bust_spinlocks.c
@@ -20,6 +20,12 @@
void __attribute__((weak)) bust_spinlocks(int yes)
{
if (yes) {
+ /*
+ * Some locks might get ignored in the Oops situation
+ * to get an important work done. Locks debug should
+ * be disabled to avoid reporting bad unlock balance.
+ */
+ debug_locks_off();
++oops_in_progress;
} else {
#ifdef CONFIG_VT