Add mutex_enter_interruptible() for interruptible sleeping IOCTLs

Many long-running ZFS ioctls lock the spa_namespace_lock, forcing
concurrent ioctls to sleep for the mutex. Previously, the only
option is to call mutex_enter() which sleeps uninterruptibly. This
is a usability issue for sysadmins, for example, if the admin runs
`zpool status` while a slow `zpool import` is ongoing, the admin's
shell will be locked in uninterruptible sleep for a long time.

This patch resolves this admin usability issue by introducing
mutex_enter_interruptible() which sleeps interruptibly while waiting
to acquire a lock. It is implemented for both Linux and FreeBSD.

The ZFS_IOC_POOL_CONFIGS ioctl, used by `zpool status`, is changed to
use this new macro so that the command can be interrupted if it is
issued during a concurrent `zpool import` (or other long-running
operation).

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Thomas Bertschinger <bertschinger@lanl.gov>
Closes #15360

module/zfs/spa_config.c

@@ -367,23 +367,24 @@ spa_write_cachefile(spa_t *target, boolean_t removing, boolean_t postsysevent,
  * So we have to invent the ZFS_IOC_CONFIG ioctl to grab the configuration
  * information for all pool visible within the zone.
  */
-nvlist_t *
-spa_all_configs(uint64_t *generation)
+int
+spa_all_configs(uint64_t *generation, nvlist_t **pools)
 {
-	nvlist_t *pools;
 	spa_t *spa = NULL;
 
 	if (*generation == spa_config_generation)
-		return (NULL);
+		return (SET_ERROR(EEXIST));
 
-	pools = fnvlist_alloc();
+	int error = mutex_enter_interruptible(&spa_namespace_lock);
+	if (error)
+		return (SET_ERROR(EINTR));
 
-	mutex_enter(&spa_namespace_lock);
+	*pools = fnvlist_alloc();
 	while ((spa = spa_next(spa)) != NULL) {
 		if (INGLOBALZONE(curproc) ||
 		    zone_dataset_visible(spa_name(spa), NULL)) {
 			mutex_enter(&spa->spa_props_lock);
-			fnvlist_add_nvlist(pools, spa_name(spa),
+			fnvlist_add_nvlist(*pools, spa_name(spa),
 			    spa->spa_config);
 			mutex_exit(&spa->spa_props_lock);
 		}
@@ -391,7 +392,7 @@ spa_all_configs(uint64_t *generation)
 	*generation = spa_config_generation;
 	mutex_exit(&spa_namespace_lock);
 
-	return (pools);
+	return (0);
 }
 
 void

module/zfs/zfs_ioctl.c

@@ -1582,8 +1582,9 @@ zfs_ioc_pool_configs(zfs_cmd_t *zc)
 	nvlist_t *configs;
 	int error;
 
-	if ((configs = spa_all_configs(&zc->zc_cookie)) == NULL)
-		return (SET_ERROR(EEXIST));
+	error = spa_all_configs(&zc->zc_cookie, &configs);
+	if (error)
+		return (error);
 
 	error = put_nvlist(zc, configs);