Re: [Xen-devel] [PATCH] libxl: make libxl_wait_for_device_model not racy

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 06/23/2010 12:15 PM, Stefano Stabellini wrote:
> Hi all,
> at the moment libxl_wait_for_device_model waits on a xenstore watch
> before checking the current value of the xenstore node, that might
> contain already the value the function was looking for.
> This patch changes libxl_wait_for_device_model so that it checks the
> value of the xenstore node first, then waits for the watch.
>   

That can't help because it's still racy: what if the value changes
between the first check and the wait?  The watch must fire immediately
if the value is already in the desired state, or there's an unavoidable
deadlock.

On the other hand, the check-then-wait pattern reads more clearly, I think.

    J

> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
>
> ---
>
> diff -r e2f5e4f3481c tools/libxl/libxl_device.c
> --- a/tools/libxl/libxl_device.c      Tue Jun 22 16:22:30 2010 +0100
> +++ b/tools/libxl/libxl_device.c      Wed Jun 23 11:04:49 2010 +0100
> @@ -418,7 +418,7 @@
>      char *path;
>      char *p;
>      unsigned int len;
> -    int rc;
> +    int rc = 0;
>      struct xs_handle *xsh;
>      int nfds;
>      fd_set rfds;
> @@ -432,28 +432,29 @@
>      tv.tv_sec = LIBXL_DEVICE_MODEL_START_TIMEOUT;
>      tv.tv_usec = 0;
>      nfds = xs_fileno(xsh) + 1;
> -    while (tv.tv_sec > 0) {
> +    while (rc > 0 || (!rc && tv.tv_sec > 0)) {
> +        p = xs_read(xsh, XBT_NULL, path, &len);
> +        if (p && (!state || !strcmp(state, p))) {
> +            free(p);
> +            xs_unwatch(xsh, path, path);
> +            xs_daemon_close(xsh);
> +            if (check_callback) {
> +                rc = check_callback(ctx, check_callback_userdata);
> +                if (rc) return rc;
> +            }
> +            return 0;
> +        }
> +        free(p);
> +again:
>          FD_ZERO(&rfds);
>          FD_SET(xs_fileno(xsh), &rfds);
> -        if (select(nfds, &rfds, NULL, NULL, &tv) > 0) {
> +        rc = select(nfds, &rfds, NULL, NULL, &tv);
> +        if (rc > 0) {
>              l = xs_read_watch(xsh, &num);
> -            if (l != NULL) {
> +            if (l != NULL)
>                  free(l);
> -                p = xs_read(xsh, XBT_NULL, path, &len);
> -                if (!p)
> -                    continue;
> -                if (!state || !strcmp(state, p)) {
> -                    free(p);
> -                    xs_unwatch(xsh, path, path);
> -                    xs_daemon_close(xsh);
> -                    if (check_callback) {
> -                        rc = check_callback(ctx, check_callback_userdata);
> -                        if (rc) return rc;
> -                    }
> -                    return 0;
> -                }
> -                free(p);
> -            }
> +            else
> +                goto again;
>          }
>      }
>      xs_unwatch(xsh, path, path);
>
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>
>   


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