Re: [PATCH v4 09/20] libmultipath: functions to indicate mapping failure in /dev/shm

["Benjamin Marzinski" <bmarzins@redhat.com>]

On Wed, Apr 04, 2018 at 06:16:16PM +0200, Martin Wilck wrote:
> Create a simple API that indicates failure to create a map for a
> certain WWID. This will allow multipathd to indicate to other tools
> (in particular, "multipath -u" during udev processing) that
> an attempt to create a map for a certain wwid failed.
> 
> The indicator is simply the existence of a file under
> /dev/shm/multipath/failed_wwids.

I'm a little confused about the necessity of a lock file here.  What it
the race that you are worried about? If two processes try to create a
file at the same time, surely one of them will succeed. If two processes
try to delete a file at the same time, it will get deleted.  If one
process is trying to create a file and one is trying to remove it, the
outcome depends on the who wins the race. But this is true whether you
add a lock file to make those actions atomic or not. The same goes with
stating a file that's being created or removed. As far a I can tell,
this should work if you simply create an empty file without any locking.
Are you worried about some odd errno due to a race?

The enums do make things less confusing, though.

-Ben

> 
> Signed-off-by: Martin Wilck <mwilck@suse.com>
> ---
>  libmultipath/defaults.h |   1 +
>  libmultipath/wwids.c    | 110 ++++++++++++++++++++++++++++++++++++++++++++++++
>  libmultipath/wwids.h    |  11 +++++
>  3 files changed, 122 insertions(+)
> 
> diff --git a/libmultipath/defaults.h b/libmultipath/defaults.h
> index 690182c..19ad2bf 100644
> --- a/libmultipath/defaults.h
> +++ b/libmultipath/defaults.h
> @@ -53,5 +53,6 @@
>  #define DEFAULT_WWIDS_FILE	"/etc/multipath/wwids"
>  #define DEFAULT_PRKEYS_FILE    "/etc/multipath/prkeys"
>  #define DEFAULT_CONFIG_DIR	"/etc/multipath/conf.d"
> +#define MULTIPATH_SHM_BASE	"/dev/shm/multipath/"
>  
>  char * set_default (char * str);
> diff --git a/libmultipath/wwids.c b/libmultipath/wwids.c
> index da685be..c9f5743 100644
> --- a/libmultipath/wwids.c
> +++ b/libmultipath/wwids.c
> @@ -5,6 +5,7 @@
>  #include <limits.h>
>  #include <stdio.h>
>  #include <sys/types.h>
> +#include <sys/stat.h>
>  
>  #include "checkers.h"
>  #include "vector.h"
> @@ -337,3 +338,112 @@ remember_wwid(char *wwid)
>  		condlog(4, "wwid %s already in wwids file", wwid);
>  	return ret;
>  }
> +
> +static const char shm_dir[] = MULTIPATH_SHM_BASE "failed_wwids";
> +static const char shm_lock[] = ".lock";
> +static const char shm_header[] = "multipath shm lock file, don't edit";
> +static char _shm_lock_path[sizeof(shm_dir)+sizeof(shm_lock)];
> +static const char *shm_lock_path = &_shm_lock_path[0];
> +
> +static void init_shm_paths(void)
> +{
> +	snprintf(_shm_lock_path, sizeof(_shm_lock_path),
> +		 "%s/%s", shm_dir, shm_lock);
> +}
> +
> +static pthread_once_t shm_path_once = PTHREAD_ONCE_INIT;
> +
> +static int multipath_shm_open(bool rw)
> +{
> +	int fd;
> +	int can_write;
> +
> +	pthread_once(&shm_path_once, init_shm_paths);
> +	fd = open_file(shm_lock_path, &can_write, shm_header);
> +
> +	if (fd >= 0 && rw && !can_write) {
> +		close(fd);
> +		condlog(1, "failed to open %s for writing", shm_dir);
> +		return -1;
> +	}
> +
> +	return fd;
> +}
> +
> +static void multipath_shm_close(void *arg)
> +{
> +	long fd = (long)arg;
> +
> +	close(fd);
> +	unlink(shm_lock_path);
> +}
> +
> +static int _failed_wwid_op(const char *wwid, bool rw,
> +			   int(*func)(const char*), const char *msg)
> +{
> +	char path[PATH_MAX];
> +	long lockfd;
> +	int r = -1;
> +
> +	if (snprintf(path, sizeof(path), "%s/%s", shm_dir, wwid)
> +	    >= sizeof(path)) {
> +		condlog(1, "%s: path name overflow", __func__);
> +		return -1;
> +	}
> +
> +	lockfd = multipath_shm_open(rw);
> +	if (lockfd == -1)
> +		return -1;
> +
> +	pthread_cleanup_push(multipath_shm_close, (void*)lockfd);
> +	r = func(path);
> +	pthread_cleanup_pop(1);
> +
> +	if (r == WWID_FAILED_ERROR)
> +		condlog(1, "%s: %s: %s", msg, wwid, strerror(errno));
> +	else if (r == WWID_FAILED_CHANGED)
> +		condlog(3, "%s: %s", msg, wwid);
> +	else if (!rw)
> +		condlog(4, "%s: %s is %s", msg, wwid,
> +			r == WWID_IS_FAILED ? "failed" : "good");
> +
> +	return r;
> +}
> +
> +static int _is_failed(const char *path)
> +{
> +	struct stat st;
> +
> +	if (lstat(path, &st) == 0)
> +		return WWID_IS_FAILED;
> +	else if (errno == ENOENT)
> +		return WWID_IS_NOT_FAILED;
> +	else
> +		return WWID_FAILED_ERROR;
> +}
> +
> +static int _mark_failed(const char *path)
> +{
> +	/* Called from _failed_wwid_op: we know that shm_lock_path exists */
> +	if (_is_failed(path) == WWID_IS_FAILED)
> +		return WWID_FAILED_UNCHANGED;
> +	return (link(shm_lock_path, path) == 0 ? WWID_FAILED_CHANGED :
> +		WWID_FAILED_ERROR);
> +}
> +
> +static int _unmark_failed(const char *path)
> +{
> +	if (_is_failed(path) == WWID_IS_NOT_FAILED)
> +		return WWID_FAILED_UNCHANGED;
> +	return (unlink(path) == 0 ? WWID_FAILED_CHANGED : WWID_FAILED_ERROR);
> +}
> +
> +#define declare_failed_wwid_op(op, rw) \
> +int op ## _wwid(const char *wwid) \
> +{ \
> +	return _failed_wwid_op(wwid, (rw), _ ## op, #op); \
> +}
> +
> +declare_failed_wwid_op(is_failed, false)
> +declare_failed_wwid_op(mark_failed, true)
> +declare_failed_wwid_op(unmark_failed, true)
> diff --git a/libmultipath/wwids.h b/libmultipath/wwids.h
> index d9a78b3..0c6ee54 100644
> --- a/libmultipath/wwids.h
> +++ b/libmultipath/wwids.h
> @@ -18,4 +18,15 @@ int check_wwids_file(char *wwid, int write_wwid);
>  int remove_wwid(char *wwid);
>  int replace_wwids(vector mp);
>  
> +enum {
> +	WWID_IS_NOT_FAILED = 0,
> +	WWID_IS_FAILED,
> +	WWID_FAILED_UNCHANGED,
> +	WWID_FAILED_CHANGED,
> +	WWID_FAILED_ERROR = -1,
> +};
> +
> +int is_failed_wwid(const char *wwid);
> +int mark_failed_wwid(const char *wwid);
> +int unmark_failed_wwid(const char *wwid);
>  #endif /* _WWIDS_H */
> -- 
> 2.16.1