Re: [PATCH 2/7] libvchan: create xenstore entries in one transaction

[David Vrabel <david.vrabel@citrix.com>]

On 21/04/13 02:46, Marek Marczykowski wrote:
> This will prevent race when client waits for server with xs_watch - all
> entries should appear at once.

Aren't xenstore writes strictly ordered?  The client just needs to read
the keys in the inverse order to avoid races, yes?

David

> 
> Signed-off-by: Marek Marczykowski <marmarek@invisiblethingslab.com>
> ---
>  tools/libvchan/init.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/libvchan/init.c b/tools/libvchan/init.c
> index f0d2505..dbdcc6c 100644
> --- a/tools/libvchan/init.c
> +++ b/tools/libvchan/init.c
> @@ -234,6 +234,7 @@ static int init_xs_srv(struct libxenvchan *ctrl, int domain, const char* xs_base
>  	char buf[64];
>  	char ref[16];
>  	char* domid_str = NULL;
> +	xs_transaction_t xs_trans = NULL;
>  	xs = xs_domain_open();
>  	if (!xs)
>  		goto fail;
> @@ -249,20 +250,29 @@ static int init_xs_srv(struct libxenvchan *ctrl, int domain, const char* xs_base
>  	perms[1].id = domain;
>  	perms[1].perms = XS_PERM_READ;
>  
> +retry_transaction:
> +	xs_trans = xs_transaction_start(xs);
> +	if (!xs_trans)
> +		goto fail_xs_open;
> +
>  	snprintf(ref, sizeof ref, "%d", ring_ref);
>  	snprintf(buf, sizeof buf, "%s/ring-ref", xs_base);
> -	if (!xs_write(xs, 0, buf, ref, strlen(ref)))
> +	if (!xs_write(xs, xs_trans, buf, ref, strlen(ref)))
>  		goto fail_xs_open;
> -	if (!xs_set_permissions(xs, 0, buf, perms, 2))
> +	if (!xs_set_permissions(xs, xs_trans, buf, perms, 2))
>  		goto fail_xs_open;
>  
>  	snprintf(ref, sizeof ref, "%d", ctrl->event_port);
>  	snprintf(buf, sizeof buf, "%s/event-channel", xs_base);
> -	if (!xs_write(xs, 0, buf, ref, strlen(ref)))
> +	if (!xs_write(xs, xs_trans, buf, ref, strlen(ref)))
>  		goto fail_xs_open;
> -	if (!xs_set_permissions(xs, 0, buf, perms, 2))
> +	if (!xs_set_permissions(xs, xs_trans, buf, perms, 2))
>  		goto fail_xs_open;
>  
> +	if (!xs_transaction_end(xs, xs_trans, 0))
> +		if (errno == EAGAIN)
> +			goto retry_transaction;
> +
>  	ret = 0;
>   fail_xs_open:
>  	free(domid_str);