From: Paolo Bonzini <pbonz...@redhat.com> This lets the read-side access run outside the BQL.
Signed-off-by: Paolo Bonzini <pbonz...@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 <pbonz...@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 -- 1.8.1.4