Re: [Xen-devel] [PATCH 09/13] libxl: introduce lock in libxl_ctx

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2011-11-28 at 16:59 +0000, Ian Jackson wrote:
> This lock will be used to protect data structures which will be hung
> off the libxl_ctx in subsequent patches.
> 
> Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>

Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

> ---
>  tools/libxl/libxl.c          |    6 ++++++
>  tools/libxl/libxl_internal.h |   27 +++++++++++++++++++++++++++
>  2 files changed, 33 insertions(+), 0 deletions(-)
> 
> diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
> index bc17b15..7488538 100644
> --- a/tools/libxl/libxl.c
> +++ b/tools/libxl/libxl.c
> @@ -41,6 +41,7 @@ int libxl_ctx_alloc(libxl_ctx **pctx, int version,
>  {
>      libxl_ctx *ctx;
>      struct stat stat_buf;
> +    const pthread_mutex_t mutex_value = 
> PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP;
>  
>      if (version != LIBXL_VERSION)
>          return ERROR_VERSION;
> @@ -54,6 +55,11 @@ int libxl_ctx_alloc(libxl_ctx **pctx, int version,
>      memset(ctx, 0, sizeof(libxl_ctx));
>      ctx->lg = lg;
>  
> +    /* This somewhat convoluted approach is needed because
> +     * PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP is defined to be valid
> +     * only as an initialiser, not as an expression. */
> +    memcpy(&ctx->lock, &mutex_value, sizeof(ctx->lock));
> +
>      if ( stat(XENSTORE_PID_FILE, &stat_buf) != 0 ) {
>          LIBXL__LOG_ERRNO(ctx, LIBXL__LOG_ERROR, "Is xenstore daemon 
> running?\n"
>                       "failed to stat %s", XENSTORE_PID_FILE);
> diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
> index cda6fc1..41de6fd 100644
> --- a/tools/libxl/libxl_internal.h
> +++ b/tools/libxl/libxl_internal.h
> @@ -23,6 +23,7 @@
>  #include <stdarg.h>
>  #include <stdlib.h>
>  #include <string.h>
> +#include <pthread.h>
>  
>  #include <xs.h>
>  #include <xenctrl.h>
> @@ -95,6 +96,18 @@ struct libxl__ctx {
>      xc_interface *xch;
>      struct xs_handle *xsh;
>  
> +    pthread_mutex_t lock; /* protects data structures hanging off the ctx */
> +      /* Always use CTX_LOCK and CTX_UNLOCK to manipulate this.
> +       *
> +       * You may acquire this mutex recursively if it is convenient to
> +       * do so.  You may not acquire this lock at the same time as any
> +       * other lock.  If you need to call application code outside
> +       * libxl (ie, a callback) with this lock held then it is
> +       * necessaray to impose restrictions on the caller to maintain a
> +       * proper lock hierarchy, and these restrictions must then be
> +       * documented in the libxl public interface.
> +       */
> +
>      /* for callers who reap children willy-nilly; caller must only
>       * set this after libxl_init and before any other call - or
>       * may leave them untouched */
> @@ -668,6 +681,20 @@ libxl__device_model_version_running(libxl__gc *gc, 
> uint32_t domid);
>  #define CTX           libxl__gc_owner(gc)
>  
> 
> +/* Locking functions.  See comment for "lock" member of libxl__ctx. */
> +
> +#define CTX_LOCK do {                                   \
> +        int mutex_r = pthread_mutex_lock(&CTX->lock);   \
> +        assert(!mutex_r);                               \
> +    } while(0)
> +
> +#define CTX_UNLOCK do {                                 \
> +        int mutex_r = pthread_mutex_unlock(&CTX->lock); \
> +        assert(!mutex_r);                               \
> +    } while(0)
> +        
> +
> +
>  /*
>   * Inserts "elm_new" into the sorted list "head".
>   *



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