From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:48229)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <jan.kiszka@siemens.com>) id 1V9A8U-0002CX-SQ
	for qemu-devel@nongnu.org; Tue, 13 Aug 2013 04:39:34 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <jan.kiszka@siemens.com>) id 1V9A8N-0004WP-GD
	for qemu-devel@nongnu.org; Tue, 13 Aug 2013 04:39:26 -0400
Received: from thoth.sbs.de ([192.35.17.2]:27388)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <jan.kiszka@siemens.com>) id 1V99wD-0008Li-Ue
	for qemu-devel@nongnu.org; Tue, 13 Aug 2013 04:26:46 -0400
Message-ID: <5209EDB0.1060500@siemens.com>
Date: Tue, 13 Aug 2013 10:26:24 +0200
From: Jan Kiszka <jan.kiszka@siemens.com>
MIME-Version: 1.0
References: <1376372630-1983-1-git-send-email-pingfank@linux.vnet.ibm.com>
	<1376372630-1983-2-git-send-email-pingfank@linux.vnet.ibm.com>
In-Reply-To: <1376372630-1983-2-git-send-email-pingfank@linux.vnet.ibm.com>
Content-Type: text/plain; charset=ISO-8859-15
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH v2 1/4] seqlock: introduce read-write
	seqlock
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Liu Ping Fan <qemulist@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, qemu-devel@nongnu.org, Alex Bligh <alex@alex.org.uk>, Paolo Bonzini <pbonzini@redhat.com>, MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>

On 2013-08-13 07:43, Liu Ping Fan wrote:
> From: Paolo Bonzini <pbonzini@redhat.com>
> 
> This lets the read-side access run outside the BQL.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  include/qemu/seqlock.h | 72 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 72 insertions(+)
>  create mode 100644 include/qemu/seqlock.h
> 
> diff --git a/include/qemu/seqlock.h b/include/qemu/seqlock.h
> new file mode 100644
> index 0000000..8f1c89f
> --- /dev/null
> +++ b/include/qemu/seqlock.h
> @@ -0,0 +1,72 @@
> +/*
> + * Seqlock implementation for QEMU
> + *
> + * Copyright Red Hat, Inc. 2013
> + *
> + * Author:
> + *  Paolo Bonzini <pbonzini@redhat.com>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + *
> + */
> +#ifndef QEMU_SEQLOCK_H
> +#define QEMU_SEQLOCK_H 1
> +
> +#include <qemu/atomic.h>
> +#include <qemu/thread.h>
> +
> +typedef struct QemuSeqLock QemuSeqLock;
> +
> +struct QemuSeqLock {
> +    QemuMutex *mutex;
> +    unsigned sequence;
> +};
> +
> +static inline void seqlock_init(QemuSeqLock *sl, QemuMutex *mutex)
> +{
> +    sl->mutex = mutex;
> +    sl->sequence = 0;
> +}
> +
> +/* Lock out other writers and update the count.  */
> +static inline void seqlock_write_lock(QemuSeqLock *sl)
> +{
> +    if (sl->mutex) {
> +        qemu_mutex_lock(sl->mutex);
> +    }
> +    ++sl->sequence;
> +
> +    /* Write sequence before updating other fields.  */
> +    smp_wmb();
> +}
> +
> +static inline void seqlock_write_unlock(QemuSeqLock *sl)
> +{
> +    /* Write other fields before finalizing sequence.  */
> +    smp_wmb();
> +
> +    ++sl->sequence;
> +    if (sl->mutex) {
> +        qemu_mutex_unlock(sl->mutex);
> +    }
> +}
> +
> +static inline unsigned seqlock_read_begin(QemuSeqLock *sl)
> +{
> +    /* Always fail if a write is in progress.  */
> +    unsigned ret = sl->sequence & ~1;
> +
> +    /* Read sequence before reading other fields.  */
> +    smp_rmb();
> +    return ret;
> +}
> +
> +static int seqlock_read_check(const QemuSeqLock *sl, unsigned start)
> +{
> +    /* Read other fields before reading final sequence.  */
> +    smp_rmb();
> +    return unlikely(sl->sequence != start);
> +}
> +
> +#endif
> 

As the usage pattern is

seqlock_read_begin()
do
	...
while (seqlock_read_check())

I would suggest to call the latter seqlock_read_retry(), just like the
kernel does.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux