Re: [Qemu-devel] [PATCH] aio: fix qemu_bh_schedule() bh->ctx race condition

[Stefan Priebe - Profihost AG <s.priebe@profihost.ag>]

Tested-by: Stefan Priebe <s.priebe@profihost.ag>

Am 03.06.2014 11:21, schrieb Stefan Hajnoczi:
> qemu_bh_schedule() is supposed to be thread-safe at least the first time
> it is called.  Unfortunately this is not quite true:
> 
>   bh->scheduled = 1;
>   aio_notify(bh->ctx);
> 
> Since another thread may run the BH callback once it has been scheduled,
> there is a race condition if the callback frees the BH before
> aio_notify(bh->ctx) has a chance to run.
> 
> Reported-by: Stefan Priebe <s.priebe@profihost.ag>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  async.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/async.c b/async.c
> index 6930185..5b6fe6b 100644
> --- a/async.c
> +++ b/async.c
> @@ -117,15 +117,21 @@ void qemu_bh_schedule_idle(QEMUBH *bh)
>  
>  void qemu_bh_schedule(QEMUBH *bh)
>  {
> +    AioContext *ctx;
> +
>      if (bh->scheduled)
>          return;
> +    ctx = bh->ctx;
>      bh->idle = 0;
> -    /* Make sure that idle & any writes needed by the callback are done
> -     * before the locations are read in the aio_bh_poll.
> +    /* Make sure that:
> +     * 1. idle & any writes needed by the callback are done before the
> +     *    locations are read in the aio_bh_poll.
> +     * 2. ctx is loaded before scheduled is set and the callback has a chance
> +     *    to execute.
>       */
> -    smp_wmb();
> +    smp_mb();
>      bh->scheduled = 1;
> -    aio_notify(bh->ctx);
> +    aio_notify(ctx);
>  }
>  
>  
>