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Date: Wed, 21 Aug 2013 10:16:02 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] block: Fix race in gluster_finish_aiocb
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To: Asias He <asias@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Vijay Bellur <vbellur@redhat.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>, Bharata B Rao <bharata@linux.vnet.ibm.com>, MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>

Il 21/08/2013 04:02, Asias He ha scritto:
> In block/gluster.c, we have
> 
> gluster_finish_aiocb
> {
>    if (retval != sizeof(acb)) {
>       qemu_mutex_lock_iothread(); /* We are in gluster thread context */
>       ...
>       qemu_mutex_unlock_iothread();
>    }
> }
> 
> qemu tools, e.g. qemu-img, might race here because
> qemu_mutex_{lock,unlock}_iothread are a nop operation and
> gluster_finish_aiocb is in the gluster thread context.
> 
> To fix, we introduce our own mutex for qemu tools.
> 
> Signed-off-by: Asias He <asias@redhat.com>
> ---
>  stubs/iothread-lock.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/stubs/iothread-lock.c b/stubs/iothread-lock.c
> index 5d8aca1..d5c6dec 100644
> --- a/stubs/iothread-lock.c
> +++ b/stubs/iothread-lock.c
> @@ -1,10 +1,21 @@
>  #include "qemu-common.h"
>  #include "qemu/main-loop.h"
>  
> +static QemuMutex qemu_tools_mutex;
> +static pthread_once_t qemu_tools_once = PTHREAD_ONCE_INIT;

Doesn't work on Windows, but you can just add
__attribute__((__constructor__)) to qemu_tools_mutex_init.

Paolo

> +static void qemu_tools_mutex_init(void)
> +{
> +    qemu_mutex_init(&qemu_tools_mutex);
> +}
> +
>  void qemu_mutex_lock_iothread(void)
>  {
> +    pthread_once(&qemu_tools_once, qemu_tools_mutex_init);
> +    qemu_mutex_lock(&qemu_tools_mutex);
>  }
>  
>  void qemu_mutex_unlock_iothread(void)
>  {
> +    qemu_mutex_unlock(&qemu_tools_mutex);
>  }
>