9079 race condition in starting and ending condesing thread for indirect vdevs

illumos/illumos-gate@667ec66f1b

The timeline of the race condition is the following:
[1] Thread A is about to finish condesing the first vdev in spa_condense_indirect_thread(),
so it calls the spa_condense_indirect_complete_sync() sync task which sets the
spa_condensing_indirect field to NULL. Waiting for the sync task to finish, thread A
sleeps until the txg is done. When this happens, thread A will acquire spa_async_lock
and set spa_condense_thread to NULL.
[2] While thread A waits for the txg to finish, thread B which is running spa_sync() checks
whether it should condense the second vdev in vdev_indirect_should_condense() by checking
the spa_condensing_indirect field which was set to NULL by spa_condense_indirect_thread()
from thread A. So it goes on and tries to spawn a new condensing thread in
spa_condense_indirect_start_sync() and the aforementioned assertions fails because thread A
has not set spa_condense_thread to NULL (which is basically the last thing it does before
returning).

The main issue here is that we rely on both spa_condensing_indirect and spa_condense_thread to
signify whether a condensing thread is running. Ideally we would only use one throughout the
codebase. In addition, for managing spa_condense_thread we currently use spa_async_lock which
basically tights condensing to scrubing when it comes to pausing and resuming those actions
during spa export.

Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Approved by: Hans Rosenfeld <rosenfeld@grumpf.hope-2000.org>
Author: Serapheim Dimitropoulos <serapheim@delphix.com>

uts/common/Makefile.files

@@ -1421,7 +1421,8 @@ ZFS_COMMON_OBJS +=		\
 	zio_compress.o		\
 	zio_inject.o		\
 	zle.o			\
-	zrlock.o
+	zrlock.o		\
+	zthr.o
 
 ZFS_SHARED_OBJS +=		\
 	zfeature_common.o	\

uts/common/fs/zfs/spa.c

@@ -1338,6 +1338,12 @@ spa_unload(spa_t *spa)
 		spa->spa_vdev_removal = NULL;
 	}
 
+	if (spa->spa_condense_zthr != NULL) {
+		ASSERT(!zthr_isrunning(spa->spa_condense_zthr));
+		zthr_destroy(spa->spa_condense_zthr);
+		spa->spa_condense_zthr = NULL;
+	}
+
 	spa_condense_fini(spa);
 
 	bpobj_close(&spa->spa_deferred_bpobj);
@@ -2079,6 +2085,16 @@ spa_vdev_err(vdev_t *vdev, vdev_aux_t aux, int err)
 	return (SET_ERROR(err));
 }
 
+static void
+spa_spawn_aux_threads(spa_t *spa)
+{
+	ASSERT(spa_writeable(spa));
+
+	ASSERT(MUTEX_HELD(&spa_namespace_lock));
+
+	spa_start_indirect_condensing_thread(spa);
+}
+
 /*
  * Fix up config after a partly-completed split.  This is done with the
  * ZPOOL_CONFIG_SPLIT nvlist.  Both the splitting pool and the split-off
@@ -3485,18 +3501,6 @@ spa_load_impl(spa_t *spa, spa_import_type_t type, char **ereport,
 
 		ASSERT(spa->spa_load_state != SPA_LOAD_TRYIMPORT);
 
-		/*
-		 * We must check this before we start the sync thread, because
-		 * we only want to start a condense thread for condense
-		 * operations that were in progress when the pool was
-		 * imported.  Once we start syncing, spa_sync() could
-		 * initiate a condense (and start a thread for it).  In
-		 * that case it would be wrong to start a second
-		 * condense thread.
-		 */
-		boolean_t condense_in_progress =
-		    (spa->spa_condensing_indirect != NULL);
-
 		/*
 		 * Traverse the ZIL and claim all blocks.
 		 */
@@ -3549,15 +3553,9 @@ spa_load_impl(spa_t *spa, spa_import_type_t type, char **ereport,
 		 */
 		dsl_pool_clean_tmp_userrefs(spa->spa_dsl_pool);
 
-		/*
-		 * Note: unlike condensing, we don't need an analogous
-		 * "removal_in_progress" dance because no other thread
-		 * can start a removal while we hold the spa_namespace_lock.
-		 */
 		spa_restart_removal(spa);
 
-		if (condense_in_progress)
-			spa_condense_indirect_restart(spa);
+		spa_spawn_aux_threads(spa);
 	}
 
 	spa_load_note(spa, "LOADED");
@@ -4466,6 +4464,8 @@ spa_create(const char *pool, nvlist_t *nvroot, nvlist_t *props,
 	 */
 	txg_wait_synced(spa->spa_dsl_pool, txg);
 
+	spa_spawn_aux_threads(spa);
+
 	spa_write_cachefile(spa, B_FALSE, B_TRUE);
 	spa_event_notify(spa, NULL, NULL, ESC_ZFS_POOL_CREATE);
 
@@ -6405,12 +6405,15 @@ spa_async_suspend(spa_t *spa)
 {
 	mutex_enter(&spa->spa_async_lock);
 	spa->spa_async_suspended++;
-	while (spa->spa_async_thread != NULL ||
-	    spa->spa_condense_thread != NULL)
+	while (spa->spa_async_thread != NULL)
 		cv_wait(&spa->spa_async_cv, &spa->spa_async_lock);
 	mutex_exit(&spa->spa_async_lock);
 
 	spa_vdev_remove_suspend(spa);
+
+	zthr_t *condense_thread = spa->spa_condense_zthr;
+	if (condense_thread != NULL && zthr_isrunning(condense_thread))
+		VERIFY0(zthr_cancel(condense_thread));
 }
 
 void
@@ -6421,6 +6424,10 @@ spa_async_resume(spa_t *spa)
 	spa->spa_async_suspended--;
 	mutex_exit(&spa->spa_async_lock);
 	spa_restart_removal(spa);
+
+	zthr_t *condense_thread = spa->spa_condense_zthr;
+	if (condense_thread != NULL && !zthr_isrunning(condense_thread))
+		zthr_resume(condense_thread);
 }
 
 static boolean_t

uts/common/fs/zfs/sys/spa_impl.h

@@ -43,6 +43,7 @@
 #include <sys/bplist.h>
 #include <sys/bpobj.h>
 #include <sys/zfeature.h>
+#include <sys/zthr.h>
 #include <zfeature_common.h>
 
 #ifdef	__cplusplus
@@ -278,7 +279,7 @@ struct spa {
 
 	spa_condensing_indirect_phys_t	spa_condensing_indirect_phys;
 	spa_condensing_indirect_t	*spa_condensing_indirect;
-	kthread_t	*spa_condense_thread;	/* thread doing condense. */
+	zthr_t		*spa_condense_zthr;	/* zthr doing condense. */
 
 	char		*spa_root;		/* alternate root directory */
 	uint64_t	spa_ena;		/* spa-wide ereport ENA */

uts/common/fs/zfs/sys/vdev_removal.h

@@ -76,7 +76,7 @@ extern int spa_remove_init(spa_t *);
 extern void spa_restart_removal(spa_t *);
 extern int spa_condense_init(spa_t *);
 extern void spa_condense_fini(spa_t *);
-extern void spa_condense_indirect_restart(spa_t *);
+extern void spa_start_indirect_condensing_thread(spa_t *);
 extern void spa_vdev_condense_suspend(spa_t *);
 extern int spa_vdev_remove(spa_t *, uint64_t, boolean_t);
 extern void free_from_removing_vdev(vdev_t *, uint64_t, uint64_t, uint64_t);

uts/common/fs/zfs/vdev_indirect.c

@@ -30,6 +30,8 @@
 #include <sys/dmu_tx.h>
 #include <sys/dsl_synctask.h>
 #include <sys/zap.h>
+#include <sys/abd.h>
+#include <sys/zthr.h>
 
 /*
  * An indirect vdev corresponds to a vdev that has been removed.  Since
@@ -475,7 +477,7 @@ spa_condense_indirect_commit_entry(spa_t *spa,
 
 static void
 spa_condense_indirect_generate_new_mapping(vdev_t *vd,
-    uint32_t *obsolete_counts, uint64_t start_index)
+    uint32_t *obsolete_counts, uint64_t start_index, zthr_t *zthr)
 {
 	spa_t *spa = vd->vdev_spa;
 	uint64_t mapi = start_index;
@@ -490,7 +492,15 @@ spa_condense_indirect_generate_new_mapping(vdev_t *vd,
 	    (u_longlong_t)vd->vdev_id,
 	    (u_longlong_t)mapi);
 
-	while (mapi < old_num_entries && !spa_shutting_down(spa)) {
+	while (mapi < old_num_entries) {
+
+		if (zthr_iscancelled(zthr)) {
+			zfs_dbgmsg("pausing condense of vdev %llu "
+			    "at index %llu", (u_longlong_t)vd->vdev_id,
+			    (u_longlong_t)mapi);
+			break;
+		}
+
 		vdev_indirect_mapping_entry_phys_t *entry =
 		    &old_mapping->vim_entries[mapi];
 		uint64_t entry_size = DVA_GET_ASIZE(&entry->vimep_dst);
@@ -508,18 +518,30 @@ spa_condense_indirect_generate_new_mapping(vdev_t *vd,
 
 		mapi++;
 	}
-	if (spa_shutting_down(spa)) {
-		zfs_dbgmsg("pausing condense of vdev %llu at index %llu",
-		    (u_longlong_t)vd->vdev_id,
-		    (u_longlong_t)mapi);
-	}
 }
 
-static void
-spa_condense_indirect_thread(void *arg)
+/* ARGSUSED */
+static boolean_t
+spa_condense_indirect_thread_check(void *arg, zthr_t *zthr)
 {
-	vdev_t *vd = arg;
-	spa_t *spa = vd->vdev_spa;
+	spa_t *spa = arg;
+
+	return (spa->spa_condensing_indirect != NULL);
+}
+
+/* ARGSUSED */
+static int
+spa_condense_indirect_thread(void *arg, zthr_t *zthr)
+{
+	spa_t *spa = arg;
+	vdev_t *vd;
+
+	ASSERT3P(spa->spa_condensing_indirect, !=, NULL);
+	spa_config_enter(spa, SCL_VDEV, FTAG, RW_READER);
+	vd = vdev_lookup_top(spa, spa->spa_condensing_indirect_phys.scip_vdev);
+	ASSERT3P(vd, !=, NULL);
+	spa_config_exit(spa, SCL_VDEV, FTAG);
+
 	spa_condensing_indirect_t *sci = spa->spa_condensing_indirect;
 	spa_condensing_indirect_phys_t *scip =
 	    &spa->spa_condensing_indirect_phys;
@@ -593,25 +615,24 @@ spa_condense_indirect_thread(void *arg)
 		}
 	}
 
-	spa_condense_indirect_generate_new_mapping(vd, counts, start_index);
+	spa_condense_indirect_generate_new_mapping(vd, counts,
+	    start_index, zthr);
 
 	vdev_indirect_mapping_free_obsolete_counts(old_mapping, counts);
 
 	/*
-	 * We may have bailed early from generate_new_mapping(), if
-	 * the spa is shutting down.  In this case, do not complete
-	 * the condense.
+	 * If the zthr has received a cancellation signal while running
+	 * in generate_new_mapping() or at any point after that, then bail
+	 * early. We don't want to complete the condense if the spa is
+	 * shutting down.
 	 */
-	if (!spa_shutting_down(spa)) {
-		VERIFY0(dsl_sync_task(spa_name(spa), NULL,
-		    spa_condense_indirect_complete_sync, sci, 0,
-		    ZFS_SPACE_CHECK_NONE));
-	}
+	if (zthr_iscancelled(zthr))
+		return (0);
 
-	mutex_enter(&spa->spa_async_lock);
-	spa->spa_condense_thread = NULL;
-	cv_broadcast(&spa->spa_async_cv);
-	mutex_exit(&spa->spa_async_lock);
+	VERIFY0(dsl_sync_task(spa_name(spa), NULL,
+	    spa_condense_indirect_complete_sync, sci, 0, ZFS_SPACE_CHECK_NONE));
+
+	return (0);
 }
 
 /*
@@ -664,9 +685,7 @@ spa_condense_indirect_start_sync(vdev_t *vd, dmu_tx_t *tx)
 	    (u_longlong_t)scip->scip_prev_obsolete_sm_object,
 	    (u_longlong_t)scip->scip_next_mapping_object);
 
-	ASSERT3P(spa->spa_condense_thread, ==, NULL);
-	spa->spa_condense_thread = thread_create(NULL, 0,
-	    spa_condense_indirect_thread, vd, 0, &p0, TS_RUN, minclsyspri);
+	zthr_wakeup(spa->spa_condense_zthr);
 }
 
 /*
@@ -743,24 +762,12 @@ spa_condense_fini(spa_t *spa)
 	}
 }
 
-/*
- * Restart the condense - called when the pool is opened.
- */
 void
-spa_condense_indirect_restart(spa_t *spa)
+spa_start_indirect_condensing_thread(spa_t *spa)
 {
-	vdev_t *vd;
-	ASSERT(spa->spa_condensing_indirect != NULL);
-	spa_config_enter(spa, SCL_VDEV, FTAG, RW_READER);
-	vd = vdev_lookup_top(spa,
-	    spa->spa_condensing_indirect_phys.scip_vdev);
-	ASSERT(vd != NULL);
-	spa_config_exit(spa, SCL_VDEV, FTAG);
-
-	ASSERT3P(spa->spa_condense_thread, ==, NULL);
-	spa->spa_condense_thread = thread_create(NULL, 0,
-	    spa_condense_indirect_thread, vd, 0, &p0, TS_RUN,
-	    minclsyspri);
+	ASSERT3P(spa->spa_condense_zthr, ==, NULL);
+	spa->spa_condense_zthr = zthr_create(spa_condense_indirect_thread_check,
+	    spa_condense_indirect_thread, spa);
 }
 
 /*