linux-kernel - Re: [PATCH] tty: add lockdep annotations

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Date:	Sat, 2 Jun 2012 18:37:47 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Alan Cox <alan@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [PATCH] tty: add lockdep annotations

On Sat, Jun 2, 2012 at 3:25 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Hmm. Ok. Looking at it, the ".shutdown" and ".remove" functions are
> all very limited, so I suspect we could just make the rule be that the
> install/lookup functions are serialized against each other by the
> pty_mutex (true today), and then we just add a small spinlock for the
> actual driver array insert/lookup.

Actually, I think we could probably make it really trivial by forcing
the free'ing of the tty itself to be RCU-delayed.

Then shutdown/remove would remove the entry with no locking
what-so-ever (which is really nice if you're in an interrupt - because
now *other* users don't need to use those annoying irq-safe versions),
and simply just clear the ttys[] array index.

The lookup side would need to just do a RCU read lock, read the
->ttys[index] thing using ACCESS_ONCE, and then just do the
atomic_inc_not_zero() dance I already did to validate that the thing
is still alive.

Voila - very cheap locking, and the part that could possibly happen
from interrupts (shutdown/remove) needs no locking at all.

Making the tty freeing be rcu-delayed sounds pretty dang simple too.

What do you think?

Anyway, I'm closing the merge window now (doing the tagging, booting
and checking that allmodconfig/allyesconfig/allnoconfigs all compile
fine) so it's 3.6 material, but it doesn't sound bad.

                Linus
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