On 02/08/2016 09:15 AM, Paolo Bonzini wrote:
> A QemuLockCnt comprises a counter and a mutex, with primitives
> to increment and decrement the counter, and to take and release the
> mutex.  It can be used to do lock-free visits to a data structure
> whenever mutexes would be too heavy-weight and the critical section
> is too long for RCU.
> 
> This could be implemented simply by protecting the counter with the
> mutex, but QemuLockCnt is harder to misuse and more efficient.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  docs/lockcnt.txt      | 343 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  include/qemu/thread.h |  17 +++
>  util/Makefile.objs    |   1 +
>  util/lockcnt.c        | 122 ++++++++++++++++++
>  4 files changed, 483 insertions(+)
>  create mode 100644 docs/lockcnt.txt
>  create mode 100644 util/lockcnt.c
> 
> diff --git a/docs/lockcnt.txt b/docs/lockcnt.txt
> new file mode 100644
> index 0000000..fc5d240
> --- /dev/null
> +++ b/docs/lockcnt.txt
> @@ -0,0 +1,343 @@
> +DOCUMENTATION FOR LOCKED COUNTERS (aka QemuLockCnt)
> +===================================================

Worth an explicit mention that this document is GPLv2+ (or an explicit
choice of a different license)?

> +
> +QEMU often uses reference counts to track data structures that are being
> +accessed and should not be freed.  For example, a loop that invoke

s/invoke/invokes/

> +callbacks like this is not safe:
> +

but overall a nice writeup.

I'll leave the code review to others, though.

> +++ b/util/lockcnt.c
> @@ -0,0 +1,122 @@
> +/*
> + * QemuLockCnt implementation
> + *
> + * Copyright Red Hat, Inc. 2015
> + *
> + * Author:
> + *   Paolo Bonzini <pbonzini@redhat.com>
> + */
> +#include <stdlib.h>
> +#include <stdio.h>
> +#include <errno.h>
> +#include <time.h>
> +#include <signal.h>
> +#include <stdint.h>
> +#include <string.h>
> +#include <limits.h>
> +#include <unistd.h>
> +#include <sys/time.h>

You'll want to use qemu/osdep.h in place of several of these headers.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org