[Libguestfs] [PATCH 2/5] threads: Acquire and release the lock around each public guestfs_* API.

[Pino Toscano]

Hi,

On Saturday 06 June 2015 14:20:38 Richard W.M. Jones wrote:
> Since each ACQUIRE_LOCK/RELEASE_LOCK call must balance, this code is
> difficult to debug.  Enable DEBUG_LOCK to add some prints which can
> help.

There's some way this could be simplified:

>  const char *
>  guestfs_last_error (guestfs_h *g)
>  {
> -  return g->last_error;
> +  const char *r;
> +
> +  ACQUIRE_LOCK (g);
> +  r = unlocked_last_error (g);
> +  RELEASE_LOCK (g);
> +  return r;
> +}

(picking this because it is the first ACQUIRE_LOCK ... RELEASE_LOCK
pattern in the patch)
there could be an extra macro to do a "stack lock", using the cleanup
attribute we use already; something like:

  static inline guestfs_h *acquire_lock (guestfs_h *g)
  {
    ACQUIRE_LOCK (g);
    return g;
  }

  static inline void cleanup_release_lock (void *ptr)
  {
    guestfs_h *g = * (guestfs_h **) ptr;

    RELEASE_LOCK (g);
  }

  #define STACK_LOCK (g) \
    __attribute__((cleanup(cleanup_release_lock))) \
      guestfs_h *_stack_lock_ = acquire_lock (g)

then the function above could look like:

  const char *
  guestfs_last_error (guestfs_h *g)
  {
    STACK_LOCK (g);
    return g->last_error;
  }

This would have the drawback of requiring __attribute__((cleanup)) to
work, although that could mean we could drop the code handling its
lack.

-- 
Pino Toscano