diff options
author | Paolo Bonzini <pbonzini@redhat.com> | 2013-09-25 14:20:57 +0800 |
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committer | Paolo Bonzini <pbonzini@redhat.com> | 2013-10-17 17:24:15 +0200 |
commit | ea753d81e8b085d679f13e4a6023e003e9854d51 (patch) | |
tree | 072bf3d6c7a3bd8726fe767cbf90470b7172ebce /include/qemu | |
parent | c46860ea53854a96b11af0d6e23b623ce199e95e (diff) | |
download | hqemu-ea753d81e8b085d679f13e4a6023e003e9854d51.zip hqemu-ea753d81e8b085d679f13e4a6023e003e9854d51.tar.gz |
seqlock: introduce read-write seqlock
Seqlock implementation for QEMU. Usage idiom
reader:
do {
start = seqlock_read_begin(&sl);
...
} while (seqlock_read_retry(&sl, start));
writer:
seqlock_write_lock(&sl);
...
seqlock_write_unlock(&sl);
initialization:
seqlock_init(QemuSeqLock *sl, QemuMutex *mutex)
mutex could be NULL if the caller will provide its own protection
for concurrent write sides (typically using the BQL).
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'include/qemu')
-rw-r--r-- | include/qemu/seqlock.h | 72 |
1 files changed, 72 insertions, 0 deletions
diff --git a/include/qemu/seqlock.h b/include/qemu/seqlock.h new file mode 100644 index 0000000..3ff118a --- /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_retry(const QemuSeqLock *sl, unsigned start) +{ + /* Read other fields before reading final sequence. */ + smp_rmb(); + return unlikely(sl->sequence != start); +} + +#endif |