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Re: [Xen-devel] [PATCH v1 02/10] libxl_internal: functions to lock / unlock domain configuration



On Thu, 2014-07-10 at 15:32 +0100, Wei Liu wrote:
> Simple file lock taken from xl to serialise access to "libxl-json" file.

Do you mean libxl here?

> If a thread cannot get hold of the lock it waits due to F_SETLKW.

How is deadlock avoided here?

I think this sort of locking is safe against processes crashing with the
lock held (it gets released), so assuming I'm right its ok from that
PoV.

> In order to generate lock file name, rename userdata_path to
> libxl__userdata_path and declare it in libxl_internal.h

Given that you don't appear to be locking the userdata file itself that
seems like an odd place for a lock to me, why not XEN_LOCK_DIR?

Did you consider locking the actual file though? That would avoid this
lock being a bottleneck...

I'm going to leave all the fctrl wrangling to Ian to look at, much more
his area of expertise.

> Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> ---
>  tools/libxl/libxl_dom.c      |   14 ++++----
>  tools/libxl/libxl_internal.c |   76 
> ++++++++++++++++++++++++++++++++++++++++++
>  tools/libxl/libxl_internal.h |   11 ++++++
>  3 files changed, 94 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/libxl/libxl_dom.c b/tools/libxl/libxl_dom.c
> index 83eb29a..a41e8fd 100644
> --- a/tools/libxl/libxl_dom.c
> +++ b/tools/libxl/libxl_dom.c
> @@ -1762,9 +1762,9 @@ char *libxl__uuid2string(libxl__gc *gc, const 
> libxl_uuid uuid)
>      return GCSPRINTF(LIBXL_UUID_FMT, LIBXL_UUID_BYTES(uuid));
>  }
>  
> -static const char *userdata_path(libxl__gc *gc, uint32_t domid,
> -                                      const char *userdata_userid,
> -                                      const char *wh)
> +const char *libxl__userdata_path(libxl__gc *gc, uint32_t domid,
> +                                 const char *userdata_userid,
> +                                 const char *wh)
>  {
>      libxl_ctx *ctx = libxl__gc_owner(gc);
>      char *uuid_string;
> @@ -1799,7 +1799,7 @@ void libxl__userdata_destroyall(libxl__gc *gc, uint32_t 
> domid)
>      glob_t gl;
>      int r, i;
>  
> -    pattern = userdata_path(gc, domid, "*", "?");
> +    pattern = libxl__userdata_path(gc, domid, "*", "?");
>      if (!pattern)
>          goto out;
>  
> @@ -1830,7 +1830,7 @@ int libxl_userdata_store(libxl_ctx *ctx, uint32_t domid,
>      int e, rc;
>      int fd = -1;
>  
> -    filename = userdata_path(gc, domid, userdata_userid, "d");
> +    filename = libxl__userdata_path(gc, domid, userdata_userid, "d");
>      if (!filename) {
>          rc = ERROR_NOMEM;
>          goto out;
> @@ -1841,7 +1841,7 @@ int libxl_userdata_store(libxl_ctx *ctx, uint32_t domid,
>          goto out;
>      }
>  
> -    newfilename = userdata_path(gc, domid, userdata_userid, "n");
> +    newfilename = libxl__userdata_path(gc, domid, userdata_userid, "n");
>      if (!newfilename) {
>          rc = ERROR_NOMEM;
>          goto out;
> @@ -1892,7 +1892,7 @@ int libxl_userdata_retrieve(libxl_ctx *ctx, uint32_t 
> domid,
>      int datalen = 0;
>      void *data = 0;
>  
> -    filename = userdata_path(gc, domid, userdata_userid, "d");
> +    filename = libxl__userdata_path(gc, domid, userdata_userid, "d");
>      if (!filename) {
>          rc = ERROR_NOMEM;
>          goto out;
> diff --git a/tools/libxl/libxl_internal.c b/tools/libxl/libxl_internal.c
> index dc47177..9cc60e8 100644
> --- a/tools/libxl/libxl_internal.c
> +++ b/tools/libxl/libxl_internal.c
> @@ -436,6 +436,82 @@ out:
>      return rc;
>  }
>  
> +int libxl__lock_domain_configuration(libxl__gc *gc, uint32_t domid,
> +                                     int *fd_lock)
> +{
> +    int rc;
> +    struct flock fl;
> +    const char *lockfile;
> +
> +    if (*fd_lock >= 0)
> +        return ERROR_INVAL;
> +
> +    lockfile = libxl__userdata_path(gc, domid, "libxl-json.lock", "d");
> +    if (!lockfile)
> +        return ERROR_FAIL;
> +
> +    *fd_lock = open(lockfile, O_WRONLY|O_CREAT, S_IWUSR);
> +    if (*fd_lock < 0) {
> +        LOGE(ERROR, "cannot open lockfile %s errno=%d\n", lockfile, errno);
> +        return ERROR_FAIL;
> +    }
> +
> +    if (fcntl(*fd_lock, F_SETFD, FD_CLOEXEC) < 0) {
> +        close(*fd_lock);
> +        LOGE(ERROR, "cannot set cloexec to lockfile %s errno=%d\n",
> +             lockfile, errno);
> +        return ERROR_FAIL;
> +    }
> +
> +get_lock:
> +    fl.l_type = F_WRLCK;
> +    fl.l_whence = SEEK_SET;
> +    fl.l_start = 0;
> +    fl.l_len = 0;
> +    rc = fcntl(*fd_lock, F_SETLKW, &fl);
> +    if (rc < 0 && errno == EINTR)
> +        goto get_lock;
> +
> +    if (rc < 0) {
> +        LOGE(ERROR, "cannot acquire lock %s errno=%d\n", lockfile, errno);
> +        rc = ERROR_FAIL;
> +    } else
> +        rc = 0;
> +
> +    return rc;
> +}
> +
> +int libxl__unlock_domain_configuration(libxl__gc *gc, uint32_t domid,
> +                                       int *fd_lock)
> +{
> +    int rc;
> +    struct flock fl;
> +
> +    if (*fd_lock < 0)
> +        return ERROR_INVAL;
> +
> +release_lock:
> +    fl.l_type = F_UNLCK;
> +    fl.l_whence = SEEK_SET;
> +    fl.l_start = 0;
> +    fl.l_len = 0;
> +    rc = fcntl(*fd_lock, F_SETLKW, &fl);
> +    if (rc < 0 && errno == EINTR)
> +        goto release_lock;
> +
> +    if (rc < 0) {
> +        const char *lockfile;
> +        lockfile = libxl__userdata_path(gc, domid, "libxl-json.lock", "d");
> +        LOGE(ERROR, "cannot release lock %s, errno=%d\n", lockfile, errno);
> +        rc = ERROR_FAIL;
> +    } else
> +        rc = 0;
> +
> +    close(*fd_lock);
> +    *fd_lock = -1;
> +
> +    return rc;
> +}
>  
>  /*
>   * Local variables:
> diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
> index ef2111b..e9d93ac 100644
> --- a/tools/libxl/libxl_internal.h
> +++ b/tools/libxl/libxl_internal.h
> @@ -991,6 +991,9 @@ _hidden int libxl__toolstack_restore(uint32_t domid, 
> const uint8_t *buf,
>                                       uint32_t size, void *data);
>  _hidden int libxl__domain_resume_device_model(libxl__gc *gc, uint32_t domid);
>  
> +_hidden const char *libxl__userdata_path(libxl__gc *gc, uint32_t domid,
> +                                         const char *userdata_userid,
> +                                         const char *wh);
>  _hidden void libxl__userdata_destroyall(libxl__gc *gc, uint32_t domid);
>  
>  _hidden int libxl__domain_resume(libxl__gc *gc, uint32_t domid,
> @@ -3228,6 +3231,14 @@ int libxl__get_domain_configuration(libxl__gc *gc, 
> uint32_t domid,
>                                      libxl_domain_config *d_config);
>  int libxl__set_domain_configuration(libxl__gc *gc, uint32_t domid,
>                                      libxl_domain_config *d_config);
> +/*
> + * Lock / unlock domain configuration in libxl private data store.
> + * fd_lock contains the file descriptor pointing to the lock file.
> + */
> +int libxl__lock_domain_configuration(libxl__gc *gc, uint32_t domid,
> +                                     int *fd_lock);
> +int libxl__unlock_domain_configuration(libxl__gc *gc, uint32_t domid,
> +                                       int *fd_lock);
>  
>  #endif
>  



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