Race condition between spa async threads and export

In the past we've seen multiple race conditions that have
to do with open-context threads async threads and concurrent
calls to spa_export()/spa_destroy() (including the one
referenced in issue #9015).

This patch ensures that only one thread can execute the
main body of spa_export_common() at a time, with subsequent
threads returning with a new error code created just for
this situation, eliminating this way any race condition
bugs introduced by concurrent calls to this function.

Reviewed by: Matt Ahrens <[email protected]>
Reviewed by: Brian Behlendorf <[email protected]>
Signed-off-by: Serapheim Dimitropoulos <[email protected]>
Closes #9015 
Closes #9044 

1 files changed, 17 insertions, 1 deletions

diff --git a/cmd/ztest/ztest.c b/cmd/ztest/ztest.c
index 3b1be5d40..01421e85b 100644
--- a/cmd/ztest/ztest.c
+++ b/cmd/ztest/ztest.c
@@ -2719,8 +2719,24 @@ ztest_spa_create_destroy(ztest_ds_t *zd, uint64_t id)
 	VERIFY3U(EEXIST, ==,
 	    spa_create(zo->zo_pool, nvroot, NULL, NULL, NULL));
 	nvlist_free(nvroot);
+
+	/*
+	 * We open a reference to the spa and then we try to export it
+	 * expecting one of the following errors:
+	 *
+	 * EBUSY
+	 *	Because of the reference we just opened.
+	 *
+	 * ZFS_ERR_EXPORT_IN_PROGRESS
+	 *	For the case that there is another ztest thread doing
+	 *	an export concurrently.
+	 */
 	VERIFY3U(0, ==, spa_open(zo->zo_pool, &spa, FTAG));
-	VERIFY3U(EBUSY, ==, spa_destroy(zo->zo_pool));
+	int error = spa_destroy(zo->zo_pool);
+	if (error != EBUSY && error != ZFS_ERR_EXPORT_IN_PROGRESS) {
+		fatal(0, "spa_destroy(%s) returned unexpected value %d",
+		    spa->spa_name, error);
+	}
 	spa_close(spa, FTAG);
 
 	(void) pthread_rwlock_unlock(&ztest_name_lock);