On Tue, Mar 16, 2021 at 11:11:43AM +0100, Krzysztof Kozlowski wrote: > On 16/03/2021 10:56, Johan Hovold wrote: > > On Tue, Mar 16, 2021 at 10:47:53AM +0100, Krzysztof Kozlowski wrote: > >> On 16/03/2021 10:02, Johan Hovold wrote: > >>> On Mon, Mar 15, 2021 at 07:12:12PM +0100, Krzysztof Kozlowski wrote: > >>>> Since interrupt handler is called with disabled local interrupts, there > >>>> is no need to use the spinlock primitives disabling interrupts as well. > >>> > >>> This isn't generally true due to "threadirqs" and that can lead to > >>> deadlocks if the console code is called from hard irq context. > >>> > >>> Now, this is *not* the case for this particular driver since it doesn't > >>> even bother to take the port lock in console_write(). That should > >>> probably be fixed instead. > >>> > >>> See https://lore.kernel.org/r/X7kviiRwuxvPxC8O@localhost. > >> > >> Thanks for the link, quite interesting! For one type of device we have > >> two interrupts (RX and TX) so I guess it's a valid point/risk. However > >> let me try to understand it more. > >> > >> Assuming we had only one interrupt line, how this interrupt handler with > >> threadirqs could be called from hardirq context? > > > > No, it's console_write() which can end up being called in hard irq > > context and if that path takes the port lock after the now threaded > > interrupt handler has been preempted you have a deadlock. > > Thanks, I understand now. I see three patterns shared by serial drivers: > > 1. Do not take the lock in console_write() handler, > 2. Take the lock like: > if (port->sysrq) > locked = 0; > else if (oops_in_progress) > locked = spin_trylock_irqsave(&port->lock, flags); > else > spin_lock_irqsave(&port->lock, flags) > > 3. Take the lock like above but preceded with local_irq_save(). > > It seems the choice of pattern depends which driver was used as a base. Right, this is messy and we've been playing whack-a-mole with this for years (as usual) it seems. Some version of 2 above is probably what we want but the sysrq bits aren't handled uniformly either (e.g. since 596f63da42b9 ("serial: 8250: Process sysrq at port unlock time")). Johan
