Re: [Xen-devel] [RFC PATCH 3/4] xl: introduce facility to run function with per-domain lock held

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Jun 14, 2017 at 06:19:20PM +0100, Wei Liu wrote:
> Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> ---
>  tools/xl/xl.h       |  1 +
>  tools/xl/xl_utils.c | 19 +++++++++++++++++++
>  tools/xl/xl_utils.h |  3 +++
>  3 files changed, 23 insertions(+)
> 
> diff --git a/tools/xl/xl.h b/tools/xl/xl.h
> index 93ec4d7e4c..8d667ff444 100644
> --- a/tools/xl/xl.h
> +++ b/tools/xl/xl.h
> @@ -292,6 +292,7 @@ extern void printf_info_sexp(int domid, 
> libxl_domain_config *d_config, FILE *fh)
>  
>  #define XL_GLOBAL_CONFIG XEN_CONFIG_DIR "/xl.conf"
>  #define XL_LOCK_FILE XEN_LOCK_DIR "/xl"
> +#define XL_DOMAIN_LOCK_FILE_FMT XEN_LOCK_DIR "/xl-%u"
>  
>  #endif /* XL_H */
>  
> diff --git a/tools/xl/xl_utils.c b/tools/xl/xl_utils.c
> index e7038ec324..bb32ba0a1f 100644
> --- a/tools/xl/xl_utils.c
> +++ b/tools/xl/xl_utils.c
> @@ -27,6 +27,25 @@
>  #include "xl.h"
>  #include "xl_utils.h"
>  
> +int with_lock(uint32_t domid, domain_fn fn, void *arg)
> +{
> +    char filename[sizeof(XL_DOMAIN_LOCK_FILE_FMT)+15];
> +    int fd_lock = -1;
> +    int rc;
> +
> +    snprintf(filename, sizeof(filename), XL_DOMAIN_LOCK_FILE_FMT, domid);
> +
> +    rc = acquire_lock(filename, &fd_lock);
> +    if (rc) goto out;
> +

It is necessary to check if the domain is still valid here. And we
should probably accept a string instead of domid in this function and
call find_domain, so that we can retry. Basically:

   retry:
       domid = find_domain();
       snprintf(...)
       rc = acquire_lock()
       if (rc) goto out;

       if (domain is not valid anymore) {
           release_lock();
           goto retry;
       }

       /* ... the rest ...*/

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