linux-kernel - Re: [PATCH v2] ARC: add smp barriers around atomics per Documentation/atomic

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Message-ID: <20150612130450.GY19282@twins.programming.kicks-ass.net>
Date:	Fri, 12 Jun 2015 15:04:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	arc-linux-dev@...opsys.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	stable@...r.kernel.org
Subject: Re: [PATCH v2] ARC: add smp barriers around atomics per
 Documentation/atomic_ops.txt

On Fri, Jun 12, 2015 at 05:45:59PM +0530, Vineet Gupta wrote:
>  - arch_spin_lock/unlock were lacking the ACQUIRE/RELEASE barriers
>    Since ARCv2 only provides load/load, store/store and all/all, we need
>    the full barrier
> 
>  - LLOCK/SCOND based atomics, bitops, cmpxchg, which return modified
>    values were lacking the explicit smp barriers.
>
>  - Non LLOCK/SCOND varaints don't need the explicit barriers since that
>    is implicity provided by the spin locks used to implement the
>    critical section (the spin lock barriers in turn are also fixed in
>    this commit as explained above

And iirc you're relying on asm-generic/barrier.h to issue
smp_mb__{before,after}_atomic() as smp_mb(), right?

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Although I'd love to know why you need those extra barriers in your
spinlocks...
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