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

[liu ping fan]

On Tue, Aug 13, 2013 at 4:26 PM, Jan Kiszka <address@hidden> wrote:
> On 2013-08-13 07:43, Liu Ping Fan wrote:
>> From: Paolo Bonzini <address@hidden>
>>
>> This lets the read-side access run outside the BQL.
>>
>> Signed-off-by: Paolo Bonzini <address@hidden>
>> ---
>>  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 <address@hidden>
>> + *
>> + * 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.
>
I think it contains the meaning of check-success-or-retry. So may
check be nicer?
>
> --
> Siemens AG, Corporate Technology, CT RTC ITP SES-DE
> Corporate Competence Center Embedded Linux