netdev - Re: [PATCH 0/3] virtio/vringh: kill off ACCESS

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Message-ID: <20161125124044.GN3092@twins.programming.kicks-ass.net>
Date:   Fri, 25 Nov 2016 13:40:44 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        linux-kernel@...r.kernel.org, dave@...olabs.net, dbueso@...e.de,
        dvyukov@...gle.com, jasowang@...hat.com, kvm@...r.kernel.org,
        netdev@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
        virtualization@...ts.linux-foundation.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()

On Fri, Nov 25, 2016 at 12:23:56PM +0000, Mark Rutland wrote:
> Naming will be problematic; calling them ATOMIC_* makes tham sound like
> they work on atomic_t. That and I have no idea how to ensure correct
> usage tree-wide; I'm not sure if/how Coccinelle can help.
> 
> Peter, thoughts?

Something like so perhaps?

---

#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
#define WARN_SINGLE_COPY_ALIGNMENT(ptr)	\
	WARN_ON_ONCE(((unsigned long)(ptr)) & (sizeof(*(ptr))-1))
#else
#define WARN_SINGLE_COPY_ALIGNMENT(ptr)
#endif

/*
 * Provide accessors for Single-Copy atomicy.
 *
 * That is, ensure that machine word sized loads/stores to naturally
 * aligned variables are single instructions.
 *
 * By reason of not being able to use C11 atomic crud, use our beloved
 * volatile qualifier. Since volatile tells the compiler the value can
 * be changed behind its back, it must use Single-Copy atomic loads and
 * stores to access them, otherwise it runs the risk of load/store
 * tearing.
 */

#define SINGLE_LOAD(x)						\
{(								\
	compiletime_assert_atomic_type(typeof(x));		\
	WARN_SINGLE_COPY_ALIGNMENT(&(x));			\
	READ_ONCE(x);						\
})

#define SINGLE_STORE(x, v)					\
({								\
	compiletime_assert_atomic_type(typeof(x));		\
	WARN_SINGLE_COPY_ALIGNMENT(&(x));			\
	WRITE_ONCE(x, v);					\
})