linux-kernel - Re: [PATCH 0.5/5] alpha: remove custom dec_and

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Date:   Wed, 6 Jun 2018 13:59:19 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        Ingo Molnar <mingo@...hat.com>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>, linux-alpha@...r.kernel.org
Subject: Re: [PATCH 0.5/5] alpha: remove custom dec_and_lock() implementation

On Wed, Jun 06, 2018 at 01:18:50PM +0200, Sebastian Andrzej Siewior wrote:

> - atomic_add_unless() adds more memory barriers compared to the custom
>   assembly

> I can't tell if the different memory barriers
> are an issue but having the same barriers is probably good.

refcount decrement needs to be a RELEASE operation, such that all the
load/stores to the object happen before we decrement the refcount.

Otherwise things like:

	obj-foo = 5;
	refcnt_dec(&obj->ref);

can be re-ordered, which then allows fun scenarios like:

	CPU0				CPU1

	refcnt_dec(&obj->ref);
					if (dec_and_test(&obj->ref))
						free(obj);
	obj->foo = 5; // oops UaF

This means (for alpha) that there should be a memory barrier _before_
the decrement, however the dec_and_lock asm thing only has one _after_,
which, per the above, is too late.

The generic version using add_unless will result in memory barrier
before and after (because that is the rule for atomic ops with a return
value) which is strictly too many barriers for the refcount story, but
who knows what other ordering requirements code has.