linux-kernel - Re: qrwlock && read-after-read

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Date:	Wed, 5 Aug 2015 15:08:23 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...hat.com>,
	"Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <Waiman.Long@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: qrwlock && read-after-read

On 08/04, Linus Torvalds wrote:
>
> I refused to have something that broke the tasklist lock, so the "irq
> users nest" was a requirement.

And I was going to reply that this breaks tasklist lock anyway but
failed to find anything wrong after the quick grep.

> So it's not like I love the current semantics, but at least they are
> realistic and can work. I agree that teaching lockdep to check for
> this would be a good idea, because the semantics _are_ subtle.

Yes... Just for example, the comment above task_lock(),

	Nests both inside and outside of read_lock(&tasklist_lock).

is no longer correct. Fortunately task_lock() is not irq-safe, and
iirc nobody does task_lock() + read_lock(&tasklist_lock) in process
context, so we are probably fine. Still, qrwlock changed the rules
and now it can only nest inside of read_lock(tasklist_lock).

Hmm. And afaics this in turn means that the next sentence

	It must not be nested with write_lock_irq(&tasklist_lock),
	neither inside nor outside.

also becomes wrong. So task_lock() can nest inside tasklist_lock,
write-or-read doesn't matter.

So it would be really nice to fix lockdep, but as Peter explains
(thanks Peter!) this is not simple.

> (And I'm not 100% convinced we needed the fair model at all, but
> fairness does end up being a good thing _if_ it works).

Yes. At least this automatically fixes the easy-to-trigger problems
with write_lock(tasklist) starvation/lockup.

Oleg.

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