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Date: Tue, 6 Aug 2013 11:30:02 +0200
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20130806093002.GA3501@stefanha-thinkpad.redhat.com>
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In-Reply-To: <1375688006-16780-3-git-send-email-pingfank@linux.vnet.ibm.com>
Subject: Re: [Qemu-devel] [PATCH 2/4] timer: protect timers_state's clock
 with seqlock
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To: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, Jan Kiszka <jan.kiszka@siemens.com>, qemu-devel@nongnu.org, Alex Bligh <alex@alex.org.uk>, Paolo Bonzini <pbonzini@redhat.com>, MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>

On Mon, Aug 05, 2013 at 03:33:24PM +0800, Liu Ping Fan wrote:
> diff --git a/cpus.c b/cpus.c
> index 85e743d..ab92db9 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -107,12 +107,17 @@ static int64_t qemu_icount;
>  typedef struct TimersState {
>      int64_t cpu_ticks_prev;
>      int64_t cpu_ticks_offset;
> +    /* QemuClock will be read out of BQL, so protect is with private lock.
> +     * As for cpu_ticks, no requirement to read it outside BQL.
> +     * Lock rule: innermost
> +     */

Please document exactly which fields the lock protects.

>  /* enable cpu_get_ticks() */
>  void cpu_enable_ticks(void)
>  {
> +    /* Here, the really thing protected by seqlock is cpu_clock. */

What is cpu_clock?

> @@ -364,6 +383,9 @@ static const VMStateDescription vmstate_timers = {
>  
>  void configure_icount(const char *option)
>  {
> +    QemuMutex *mutex = g_malloc0(sizeof(QemuMutex));
> +    qemu_mutex_init(mutex);
> +    seqlock_init(&timers_state.clock_seqlock, mutex);

We always set up this mutex, so it could be a field in timers_state.
That avoids the g_malloc() without g_free().