linux-kernel - Re: [PATCH 1/2] ipc/sem.c: Fix complex

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Date:	Fri, 1 Jul 2016 09:52:58 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Stephen Rothwell <sfr@...b.auug.org.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-next@...r.kernel.org,
	1vier1@....de, felixh@...ormatik.uni-bremen.de,
	stable@...r.kernel.org
Subject: Re: [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race

On Thu, 30 Jun 2016, Manfred Spraul wrote:

>On 06/28/2016 07:27 AM, Davidlohr Bueso wrote:

>If I understand it right, it means that spin_lock() is both an acquire 
>and a release - for qspinlocks.

I wouldn't say that: the _Q_LOCKED_VAL stores are still unordered inside
the acquire region but no further than the the unlock store-release; which
is fine for regular mutual exclusion.

>It this valid for all spinlock implementations, for all architectures?
>Otherwise: How can I detect in generic code if I can rely on a release 
>inside spin_lock()?

With ticket locks this was never an issue as special lock readers
(spin_unlock_wait(), spin_is_locked()) will always see the lock taken as
they observe waiters by adding itself to the tail -- something the above
commit mimics and piggy backs on to save expensive smp_rmb().

In addition, see 726328d92a4 (locking/spinlock, arch: Update and fix
spin_unlock_wait() implementations).

Thanks,
Davidlohr