Date: Thu, 10 May 2012 14:04:00 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
cc: "H. Peter Anvin" <hpa@zytor.com>, Rob Landley <rob@landley.net>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        x86@kernel.org, Arnd Bergmann <arnd@arndb.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Howells <dhowells@redhat.com>,
        Akinobu Mita <akinobu.mita@gmail.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Herbert Xu <herbert@gondor.hengli.com.au>,
        Stephen Rothwell <sfr@canb.auug.org.au>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        Gleb Natapov <gleb@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>,
        kvm@vger.kernel.org, Avi Kivity <avi@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCHv2] bitops: add _local bitops
In-Reply-To: <20120509162819.GA21328@redhat.com>
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References: <20120509162819.GA21328@redhat.com>
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On Wed, 9 May 2012, Michael S. Tsirkin wrote:

> kvm needs to update some hypervisor variables atomically
> in a sense that the operation can't be interrupted
> in the middle. However the hypervisor always runs
> on the same CPU so it does not need any memory
> barrier or lock prefix.
>
> Add _local bitops for this purpose: define them
> as non-atomics for x86 and (for now) atomics for
> everyone else.

Have you tried to use the this_cpu_ops for that purpose? They create the
per cpu atomic instructions that you want without a lock prefix and can
also relocate the per cpu pointer to the correct processor via a
segment register prefix.

There are no bit operations provided right now but those can either be
improvised using this_cpu_cmpxchg or added.
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