Re: [Qemu-devel] [PATCH v2 1/4] seqlock: introduce read-write seqlock

[Jan Kiszka <jan.kiszka@siemens.com>]

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

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