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SubjectRe: [PATCHv3] panic: avoid deadlocks in re-entrant console drivers
On Thu 2018-10-25 19:10:36, 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.
>
> 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().
>
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

The patch makes sense to me. The locks should stay busted also for
console_flush_on_panic().

With the added #include <linux/vt_kern.h>:

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr

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