Re: [Qemu-devel] [PATCH 02/10] qemu-thread: introduce QemuLockCnt

[Fam Zheng <famz@redhat.com>]

On Wed, 01/04 14:26, Paolo Bonzini wrote:
> +For example, QEMU uses QemuLockCnt to manage an AioContext's list of
> +bottom halves and file descriptor handlers.  Modifications to the list
> +of file descriptor handlers are rare.  Creation of a new bottom half is
> +frequent and can happen on a fast path; however: 1) it is almost never
> +concurrent with a visit to the list of bottom halves; 

Isn't it common that thread A is busy notifying thread B with BH, and thread B
is busy processing the notification? In that case creation and visiting the BH
list are concurrent.

> 2) it only has
> +three instructions in the critical path, two assignments and a smp_wmb().
> +
> +/**
> + * qemu_lockcnt_dec: possibly decrement a QemuLockCnt's counter and lock it.

s/qemu_lockcnt_dec/qemu_lockcnt_dec_if_lock/

> + * @lockcnt: the lockcnt to operate on
> + *
> + * If the count is 1, decrement the count to zero, lock
> + * the mutex and return true.  Otherwise, return false.
> + */
> +bool qemu_lockcnt_dec_if_lock(QemuLockCnt *lockcnt);
> +
> +/**
> + * qemu_lockcnt_lock: lock a QemuLockCnt's mutex.
> + * @lockcnt: the lockcnt to operate on
> + *
> + * Remember that concurrent visits are not blocked unless the count is
> + * also zero.  You can use qemu_lockcnt_count to check for this inside a
> + * critical section.
> + */
> +void qemu_lockcnt_lock(QemuLockCnt *lockcnt);
> diff --git a/util/lockcnt.c b/util/lockcnt.c
> new file mode 100644
> index 0000000..78ed1e4
> --- /dev/null
> +++ b/util/lockcnt.c
> @@ -0,0 +1,113 @@
> +/*
> + * QemuLockCnt implementation
> + *
> + * Copyright Red Hat, Inc. 2015

2015, 2016?

> + *
> + * Author:
> + *   Paolo Bonzini <pbonzini@redhat.com>
> + */
> +#include "qemu/osdep.h"
> +#include "qemu/thread.h"
> +#include "qemu/atomic.h"