Hi, crbug.com/1114800 reports a hard lockup due to circular locking in the 8250 console driver. This is seen if CONFIG_PROVE_LOCKING is enabled. Problem is as follows: - serial8250_do_startup() locks the serial (console) port. - serial8250_do_startup() then disables interrupts if interrupts are shared, by calling disable_irq_nosync(). - disable_irq_nosync() calls __irq_get_desc_lock() to lock the interrupt descriptor. - __irq_get_desc_lock() calls lock_acquire() - If CONFIG_PROVE_LOCKING is enabled, validate_chain() and check_noncircular() are called and identify a potential locking error. - This locking error is reported via printk, which ultimately calls serial8250_console_write(). - serial8250_console_write() tries to lock the serial console port. Since it is already locked, the system hangs and ultimately reports a hard lockup. I understand we'll need to figure out and fix what lockdep complains about, and I am working on that. However, even if that is fixed, we'll need a solution for the recursive lock: Fixing the lockdep problem doesn't guarantee that a similar problem (or some other log message) won't be detected and reported sometime in the future while serial8250_do_startup() holds the console port lock. Ideas, anyone ? Everything I came up with so far seems clumsy and hackish. Thanks, Guenter
