Re: [RFC][PATCH 0/6] Use printk_safe context for TTY and UART port locks

[Sergey Senozhatsky]

On (06/20/18 12:38), Linus Torvalds wrote:
> On Wed, Jun 20, 2018 at 11:50 AM Sergey Senozhatsky
> <sergey.senozhatsky.work@xxxxxxxxx> wrote:
> >
> > It's not UART on its own that immediately calls into printk(), that would
> > be trivial to fix, it's all those subsystems that serial console driver
> > can call into.
> 
> We already have the whole PRINTK_SAFE_CONTEXT_MASK model that only
> adds it to a secondary buffer if you get recursion.  Why isn't that
> triggering? That's the whole point of it.

This is exactly what I'm doing in my patch set.
PRINTK_SAFE_CONTEXT_MASK so far worked *one* way only: when we start
from printk.c

IOW:

      printk -> printk_safe_mask -> vsprinf -> printk

But we also can have printk-related deadlocks the *other* way
around. For instance:

      uart -> printk -> uart

printk_safe_mask is not triggering there because we don't use
printk_safe in uart / tty yet. And this is what I do in my
patch set - extend printk_safe usage.

The patch set does not add any _new_ locks or locking rules.
It just replaces the existing

		spin_lock(a)
with
		prinkt_safe_enter();
		spin_lock(a)

and
		spin_unlock(a)
with
		spin_unlock(a)
		printk_safe_exit();

and that's it.

So now we use printk_safe mechanism to avoid another bunch of
deadlock scenarious: which don't start from printk, but from
parts of the kernel which printk eventually calls.

	-ss