On Mon, Jul 3, 2017 at 7:24 AM, Petr Mladek <[email protected]> wrote:
>
> - Store printk() messages into the main log buffer directly even in NMI
> when the lock is available. It is the best effort to print even large
> chunk of text. It is handy, for example, when all ftrace messages
> are printed during the system panic in NMI.
No, this is entirely buggered.
You can't just do
raw_spin_is_locked(&logbuf_lock)
to test whether you can get the logbuf_lock and then call vprintk_default().
It's not just about deadlock avoidance, the code will call things like
down_trylock() on the console semaphore, and that operation is
fundamentally not NMI-safe. The semaphore count is literally protected
by a irq-safe (BUT NOT NMI-SAFE!) semaphore spinlock.
So now you can instead deadlock just on the internal console semaphore
spinlock (ie somebody is doing "console_lock()", NMI comes in, *BOOM*.
Maybe I'm missing something, but I'm skipping this pull request as
"completely broken".
Linus
