On (11/02/17 17:53), Sergey Senozhatsky wrote:
> On (10/31/17 15:32), Steven Rostedt wrote:
> [..]
> > (new globals)
> > static DEFINE_SPIN_LOCK(console_owner_lock);
> > static struct task_struct console_owner;
> > static bool waiter;
> >
> > console_unlock() {
> >
> > [ Assumes this part can not preempt ]
> >
> > spin_lock(console_owner_lock);
> > console_owner = current;
> > spin_unlock(console_owner_lock);
>
> + disables IRQs?
>
> > for each message
> > write message out to console
> >
> > if (READ_ONCE(waiter))
> > break;
> >
> > spin_lock(console_owner_lock);
> > console_owner = NULL;
> > spin_unlock(console_owner_lock);
> >
> > [ preemption possible ]
>
> otherwise
>
> printk()
> if (console_trylock())
> console_unlock()
> preempt_disable()
> spin_lock(console_owner_lock);
> console_owner = current;
> spin_unlock(console_owner_lock);
> .......
> spin_lock(console_owner_lock);
> IRQ
> printk()
> console_trylock() // fails so we go to busy-loop part
> spin_lock(console_owner_lock); << deadlock
>
>
> even if we would replace spin_lock(console_owner_lock) with IRQ
> spin_lock, we still would need to protect against IRQs on the very
> same CPU. right? IOW, we need to store smp_processor_id() of a CPU
> currently doing console_unlock() and check it in vprintk_emit()?
a major self-correction:
> and we need to protect the entire console_unlock() function. not
> just the printing loop, otherwise the IRQ CPU will spin forever
> waiting for itself to up() the console_sem.
this part is wrong. should have been
"we need to protect the entire printing loop"
so now console_unlock()'s printing loop is going to run
a) under preempt_disable()
b) under local_irq_save()
which is risky.
-ss
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