Serialize ZTHR operations to eliminate races

Adds a new lock for serializing operations on zthrs.
The commit also includes some code cleanup and
refactoring.

Reviewed by: Matt Ahrens <[email protected]>
Reviewed by: Tom Caputi <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Serapheim Dimitropoulos <[email protected]>
Closes #8229 

5 files changed, 188 insertions, 110 deletions

diff --git a/module/zfs/arc.c b/module/zfs/arc.c
index 9e0ffd06d..7e0963334 100644
--- a/module/zfs/arc.c
+++ b/module/zfs/arc.c
@@ -5113,7 +5113,7 @@ arc_adjust_cb_check(void *arg, zthr_t *zthr)
  * from the ARC.
  */
 /* ARGSUSED */
-static int
+static void
 arc_adjust_cb(void *arg, zthr_t *zthr)
 {
 	uint64_t evicted = 0;
@@ -5147,8 +5147,6 @@ arc_adjust_cb(void *arg, zthr_t *zthr)
 	}
 	mutex_exit(&arc_adjust_lock);
 	spl_fstrans_unmark(cookie);
-
-	return (0);
 }
 
 /* ARGSUSED */
@@ -5190,7 +5188,7 @@ arc_reap_cb_check(void *arg, zthr_t *zthr)
  * to free more buffers.
  */
 /* ARGSUSED */
-static int
+static void
 arc_reap_cb(void *arg, zthr_t *zthr)
 {
 	int64_t free_memory;
@@ -5231,8 +5229,6 @@ arc_reap_cb(void *arg, zthr_t *zthr)
 		arc_reduce_target_size(to_free);
 	}
 	spl_fstrans_unmark(cookie);
-
-	return (0);
 }
 
 #ifdef _KERNEL
diff --git a/module/zfs/spa.c b/module/zfs/spa.c
index c4ff5002b..c98daab49 100644
--- a/module/zfs/spa.c
+++ b/module/zfs/spa.c
@@ -1486,13 +1486,11 @@ spa_unload(spa_t *spa)
 	}
 
 	if (spa->spa_condense_zthr != NULL) {
-		ASSERT(!zthr_isrunning(spa->spa_condense_zthr));
 		zthr_destroy(spa->spa_condense_zthr);
 		spa->spa_condense_zthr = NULL;
 	}
 
 	if (spa->spa_checkpoint_discard_zthr != NULL) {
-		ASSERT(!zthr_isrunning(spa->spa_checkpoint_discard_zthr));
 		zthr_destroy(spa->spa_checkpoint_discard_zthr);
 		spa->spa_checkpoint_discard_zthr = NULL;
 	}
@@ -7214,12 +7212,12 @@ spa_async_suspend(spa_t *spa)
 	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));
+	if (condense_thread != NULL)
+		zthr_cancel(condense_thread);
 
 	zthr_t *discard_thread = spa->spa_checkpoint_discard_zthr;
-	if (discard_thread != NULL && zthr_isrunning(discard_thread))
-		VERIFY0(zthr_cancel(discard_thread));
+	if (discard_thread != NULL)
+		zthr_cancel(discard_thread);
 }
 
 void
@@ -7232,11 +7230,11 @@ spa_async_resume(spa_t *spa)
 	spa_restart_removal(spa);
 
 	zthr_t *condense_thread = spa->spa_condense_zthr;
-	if (condense_thread != NULL && !zthr_isrunning(condense_thread))
+	if (condense_thread != NULL)
 		zthr_resume(condense_thread);
 
 	zthr_t *discard_thread = spa->spa_checkpoint_discard_zthr;
-	if (discard_thread != NULL && !zthr_isrunning(discard_thread))
+	if (discard_thread != NULL)
 		zthr_resume(discard_thread);
 }
 
diff --git a/module/zfs/spa_checkpoint.c b/module/zfs/spa_checkpoint.c
index 863ec46b1..230ae5785 100644
--- a/module/zfs/spa_checkpoint.c
+++ b/module/zfs/spa_checkpoint.c
@@ -393,7 +393,7 @@ spa_checkpoint_discard_thread_check(void *arg, zthr_t *zthr)
 	return (B_TRUE);
 }
 
-int
+void
 spa_checkpoint_discard_thread(void *arg, zthr_t *zthr)
 {
 	spa_t *spa = arg;
@@ -408,7 +408,7 @@ spa_checkpoint_discard_thread(void *arg, zthr_t *zthr)
 			dmu_buf_t **dbp;
 
 			if (zthr_iscancelled(zthr))
-				return (0);
+				return;
 
 			ASSERT3P(vd->vdev_ops, !=, &vdev_indirect_ops);
 
@@ -445,8 +445,6 @@ spa_checkpoint_discard_thread(void *arg, zthr_t *zthr)
 	VERIFY0(dsl_sync_task(spa->spa_name, NULL,
 	    spa_checkpoint_discard_complete_sync, spa,
 	    0, ZFS_SPACE_CHECK_NONE));
-
-	return (0);
 }
 
 
diff --git a/module/zfs/vdev_indirect.c b/module/zfs/vdev_indirect.c
index 070d1b8d9..d0725add8 100644
--- a/module/zfs/vdev_indirect.c
+++ b/module/zfs/vdev_indirect.c
@@ -647,7 +647,7 @@ spa_condense_indirect_thread_check(void *arg, zthr_t *zthr)
 }
 
 /* ARGSUSED */
-static int
+static void
 spa_condense_indirect_thread(void *arg, zthr_t *zthr)
 {
 	spa_t *spa = arg;
@@ -744,13 +744,11 @@ spa_condense_indirect_thread(void *arg, zthr_t *zthr)
 	 * shutting down.
 	 */
 	if (zthr_iscancelled(zthr))
-		return (0);
+		return;
 
 	VERIFY0(dsl_sync_task(spa_name(spa), NULL,
 	    spa_condense_indirect_complete_sync, sci, 0,
 	    ZFS_SPACE_CHECK_EXTRA_RESERVED));
-
-	return (0);
 }
 
 /*
diff --git a/module/zfs/zthr.c b/module/zfs/zthr.c
index c5b11dafc..64372f338 100644
--- a/module/zfs/zthr.c
+++ b/module/zfs/zthr.c
@@ -14,7 +14,7 @@
  */
 
 /*
- * Copyright (c) 2017 by Delphix. All rights reserved.
+ * Copyright (c) 2017, 2019 by Delphix. All rights reserved.
  */
 
 /*
@@ -28,7 +28,7 @@
  *
  * 1] The operation needs to run over multiple txgs.
  * 2] There is be a single point of reference in memory or on disk that
- *    indicates whether the operation should run/is running or is
+ *    indicates whether the operation should run/is running or has
  *    stopped.
  *
  * If the operation satisfies the above then the following rules guarantee
@@ -51,6 +51,9 @@
  * during creation to wakeup on its own after a specified interval
  * [see zthr_create_timer()].
  *
+ * Note: ZTHR threads are NOT a replacement for generic threads! Please
+ * ensure that they fit your use-case well before using them.
+ *
  * == ZTHR creation
  *
  * Every zthr needs three inputs to start running:
@@ -64,17 +67,17 @@
  * 2] A user-defined ZTHR function (func) which the zthr executes when
  *    it is not sleeping. The function should adhere to the following
  *    signature type:
- *    int func_name(void *args, zthr_t *t);
+ *    void func_name(void *args, zthr_t *t);
  *
  * 3] A void args pointer that will be passed to checkfunc and func
  *    implicitly by the infrastructure.
  *
  * The reason why the above API needs two different functions,
  * instead of one that both checks and does the work, has to do with
- * the zthr's internal lock (zthr_lock) and the allowed cancellation
- * windows. We want to hold the zthr_lock while running checkfunc
- * but not while running func. This way the zthr can be cancelled
- * while doing work and not while checking for work.
+ * the zthr's internal state lock (zthr_state_lock) and the allowed
+ * cancellation windows. We want to hold the zthr_state_lock while
+ * running checkfunc but not while running func. This way the zthr
+ * can be cancelled while doing work and not while checking for work.
  *
  * To start a zthr:
  *     zthr_t *zthr_pointer = zthr_create(checkfunc, func, args);
@@ -83,7 +86,7 @@
  *         args, max_sleep);
  *
  * After that you should be able to wakeup, cancel, and resume the
- * zthr from another thread using zthr_pointer.
+ * zthr from another thread using the zthr_pointer.
  *
  * NOTE: ZTHR threads could potentially wake up spuriously and the
  * user should take this into account when writing a checkfunc.
@@ -102,8 +105,8 @@
  *     zthr_resume(zthr_pointer);
  *
  * A zthr will implicitly check if it has received a cancellation
- * signal every time func returns and everytime it wakes up [see ZTHR
- * state transitions below].
+ * signal every time func returns and every time it wakes up [see
+ * ZTHR state transitions below].
  *
  * At times, waiting for the zthr's func to finish its job may take
  * time. This may be very time-consuming for some operations that
@@ -119,17 +122,8 @@
  *     while (!work_done && !zthr_iscancelled(t)) {
  *         ... <do more work> ...
  *     }
- *     return (0);
  * }
  *
- * == ZTHR exit
- *
- * For the rare cases where the zthr wants to stop running voluntarily
- * while running its ZTHR function (func), we provide zthr_exit().
- * When a zthr has voluntarily stopped running, it can be resumed with
- * zthr_resume(), just like it would if it was cancelled by some other
- * thread.
- *
  * == ZTHR cleanup
  *
  * Cancelling a zthr doesn't clean up its metadata (internal locks,
@@ -165,49 +159,86 @@
  *      v
  *   zthr stopped running
  *
+ * == Implementation of ZTHR requests
+ *
+ * ZTHR wakeup, cancel, and resume are requests on a zthr to
+ * change its internal state. Requests on a zthr are serialized
+ * using the zthr_request_lock, while changes in its internal
+ * state are protected by the zthr_state_lock. A request will
+ * first acquire the zthr_request_lock and then immediately
+ * acquire the zthr_state_lock. We do this so that incoming
+ * requests are serialized using the request lock, while still
+ * allowing us to use the state lock for thread communication
+ * via zthr_cv.
  */
 
 #include <sys/zfs_context.h>
 #include <sys/zthr.h>
 
-void
-zthr_exit(zthr_t *t, int rc)
-{
-	ASSERT3P(t->zthr_thread, ==, curthread);
-	mutex_enter(&t->zthr_lock);
-	t->zthr_thread = NULL;
-	t->zthr_rc = rc;
-	cv_broadcast(&t->zthr_cv);
-	mutex_exit(&t->zthr_lock);
-	thread_exit();
-}
+struct zthr {
+	/* running thread doing the work */
+	kthread_t	*zthr_thread;
+
+	/* lock protecting internal data & invariants */
+	kmutex_t	zthr_state_lock;
+
+	/* mutex that serializes external requests */
+	kmutex_t	zthr_request_lock;
+
+	/* notification mechanism for requests */
+	kcondvar_t	zthr_cv;
+
+	/* flag set to true if we are canceling the zthr */
+	boolean_t	zthr_cancel;
+
+	/*
+	 * maximum amount of time that the zthr is spent sleeping;
+	 * if this is 0, the thread doesn't wake up until it gets
+	 * signaled.
+	 */
+	hrtime_t	zthr_wait_time;
+
+	/* consumer-provided callbacks & data */
+	zthr_checkfunc_t	*zthr_checkfunc;
+	zthr_func_t	*zthr_func;
+	void		*zthr_arg;
+};
 
 static void
 zthr_procedure(void *arg)
 {
 	zthr_t *t = arg;
-	int rc = 0;
 
-	mutex_enter(&t->zthr_lock);
+	mutex_enter(&t->zthr_state_lock);
+	ASSERT3P(t->zthr_thread, ==, curthread);
+
 	while (!t->zthr_cancel) {
 		if (t->zthr_checkfunc(t->zthr_arg, t)) {
-			mutex_exit(&t->zthr_lock);
-			rc = t->zthr_func(t->zthr_arg, t);
-			mutex_enter(&t->zthr_lock);
+			mutex_exit(&t->zthr_state_lock);
+			t->zthr_func(t->zthr_arg, t);
+			mutex_enter(&t->zthr_state_lock);
 		} else {
 			/* go to sleep */
 			if (t->zthr_wait_time == 0) {
-				cv_wait_sig(&t->zthr_cv, &t->zthr_lock);
+				cv_wait_sig(&t->zthr_cv, &t->zthr_state_lock);
 			} else {
 				(void) cv_timedwait_sig_hires(&t->zthr_cv,
-				    &t->zthr_lock, t->zthr_wait_time,
+				    &t->zthr_state_lock, t->zthr_wait_time,
 				    MSEC2NSEC(1), 0);
 			}
 		}
 	}
-	mutex_exit(&t->zthr_lock);
 
-	zthr_exit(t, rc);
+	/*
+	 * Clear out the kernel thread metadata and notify the
+	 * zthr_cancel() thread that we've stopped running.
+	 */
+	t->zthr_thread = NULL;
+	t->zthr_cancel = B_FALSE;
+	cv_broadcast(&t->zthr_cv);
+
+	mutex_exit(&t->zthr_state_lock);
+	thread_exit();
 }
 
 zthr_t *
@@ -226,10 +257,11 @@ zthr_create_timer(zthr_checkfunc_t *checkfunc, zthr_func_t *func,
     void *arg, hrtime_t max_sleep)
 {
 	zthr_t *t = kmem_zalloc(sizeof (*t), KM_SLEEP);
-	mutex_init(&t->zthr_lock, NULL, MUTEX_DEFAULT, NULL);
+	mutex_init(&t->zthr_state_lock, NULL, MUTEX_DEFAULT, NULL);
+	mutex_init(&t->zthr_request_lock, NULL, MUTEX_DEFAULT, NULL);
 	cv_init(&t->zthr_cv, NULL, CV_DEFAULT, NULL);
 
-	mutex_enter(&t->zthr_lock);
+	mutex_enter(&t->zthr_state_lock);
 	t->zthr_checkfunc = checkfunc;
 	t->zthr_func = func;
 	t->zthr_arg = arg;
@@ -237,7 +269,7 @@ zthr_create_timer(zthr_checkfunc_t *checkfunc, zthr_func_t *func,
 
 	t->zthr_thread = thread_create(NULL, 0, zthr_procedure, t,
 	    0, &p0, TS_RUN, minclsyspri);
-	mutex_exit(&t->zthr_lock);
+	mutex_exit(&t->zthr_state_lock);
 
 	return (t);
 }
@@ -245,71 +277,130 @@ zthr_create_timer(zthr_checkfunc_t *checkfunc, zthr_func_t *func,
 void
 zthr_destroy(zthr_t *t)
 {
+	ASSERT(!MUTEX_HELD(&t->zthr_state_lock));
+	ASSERT(!MUTEX_HELD(&t->zthr_request_lock));
 	VERIFY3P(t->zthr_thread, ==, NULL);
-	mutex_destroy(&t->zthr_lock);
+	mutex_destroy(&t->zthr_request_lock);
+	mutex_destroy(&t->zthr_state_lock);
 	cv_destroy(&t->zthr_cv);
 	kmem_free(t, sizeof (*t));
 }
 
 /*
- * Note: If the zthr is not sleeping and misses the wakeup
- * (e.g it is running its ZTHR function), it will check if
- * there is work to do before going to sleep using its checker
- * function [see ZTHR state transition in ZTHR block comment].
- * Thus, missing the wakeup still yields the expected behavior.
+ * Wake up the zthr if it is sleeping. If the thread has been
+ * cancelled that does nothing.
  */
 void
 zthr_wakeup(zthr_t *t)
 {
-	mutex_enter(&t->zthr_lock);
+	mutex_enter(&t->zthr_request_lock);
+	mutex_enter(&t->zthr_state_lock);
+
+	/*
+	 * There are 4 states that we can find the zthr when issuing
+	 * this broadcast:
+	 *
+	 * [1] The common case of the thread being asleep, at which
+	 *     point the broadcast will wake it up.
+	 * [2] The thread has been cancelled. Waking up a cancelled
+	 *     thread is a no-op. Any work that is still left to be
+	 *     done should be handled the next time the thread is
+	 *     resumed.
+	 * [3] The thread is doing work and is already up, so this
+	 *     is basically a no-op.
+	 * [4] The thread was just created/resumed, in which case the
+	 *     behavior is similar to [3].
+	 */
 	cv_broadcast(&t->zthr_cv);
-	mutex_exit(&t->zthr_lock);
+
+	mutex_exit(&t->zthr_state_lock);
+	mutex_exit(&t->zthr_request_lock);
 }
 
 /*
- * Note: If the zthr is not running (e.g. has been cancelled
+ * Sends a cancel request to the zthr and blocks until the zthr is
+ * cancelled. If the zthr is not running (e.g. has been cancelled
  * already), this is a no-op.
  */
-int
+void
 zthr_cancel(zthr_t *t)
 {
-	int rc = 0;
+	mutex_enter(&t->zthr_request_lock);
+	mutex_enter(&t->zthr_state_lock);
 
-	mutex_enter(&t->zthr_lock);
+	/*
+	 * Since we are holding the zthr_state_lock at this point
+	 * we can find the state in one of the following 4 states:
+	 *
+	 * [1] The thread has already been cancelled, therefore
+	 *     there is nothing for us to do.
+	 * [2] The thread is sleeping, so we broadcast the CV first
+	 *     to wake it up and then we set the flag and we are
+	 *     waiting for it to exit.
+	 * [3] The thread is doing work, in which case we just set
+	 *     the flag and wait for it to finish.
+	 * [4] The thread was just created/resumed, in which case
+	 *     the behavior is similar to [3].
+	 *
+	 * Since requests are serialized, by the time that we get
+	 * control back we expect that the zthr is cancelled and
+	 * not running anymore.
+	 */
+	if (t->zthr_thread != NULL) {
+		t->zthr_cancel = B_TRUE;
 
-	/* broadcast in case the zthr is sleeping */
-	cv_broadcast(&t->zthr_cv);
+		/* broadcast in case the zthr is sleeping */
+		cv_broadcast(&t->zthr_cv);
 
-	t->zthr_cancel = B_TRUE;
-	while (t->zthr_thread != NULL)
-		cv_wait(&t->zthr_cv, &t->zthr_lock);
-	t->zthr_cancel = B_FALSE;
-	rc = t->zthr_rc;
-	mutex_exit(&t->zthr_lock);
+		while (t->zthr_thread != NULL)
+			cv_wait(&t->zthr_cv, &t->zthr_state_lock);
 
-	return (rc);
+		ASSERT(!t->zthr_cancel);
+	}
+
+	mutex_exit(&t->zthr_state_lock);
+	mutex_exit(&t->zthr_request_lock);
 }
 
+/*
+ * Sends a resume request to the supplied zthr. If the zthr is
+ * already running this is a no-op.
+ */
 void
 zthr_resume(zthr_t *t)
 {
-	ASSERT3P(t->zthr_thread, ==, NULL);
-
-	mutex_enter(&t->zthr_lock);
+	mutex_enter(&t->zthr_request_lock);
+	mutex_enter(&t->zthr_state_lock);
 
 	ASSERT3P(&t->zthr_checkfunc, !=, NULL);
 	ASSERT3P(&t->zthr_func, !=, NULL);
 	ASSERT(!t->zthr_cancel);
 
-	t->zthr_thread = thread_create(NULL, 0, zthr_procedure, t,
-	    0, &p0, TS_RUN, minclsyspri);
+	/*
+	 * There are 4 states that we find the zthr in at this point
+	 * given the locks that we hold:
+	 *
+	 * [1] The zthr was cancelled, so we spawn a new thread for
+	 *     the zthr (common case).
+	 * [2] The zthr is running at which point this is a no-op.
+	 * [3] The zthr is sleeping at which point this is a no-op.
+	 * [4] The zthr was just spawned at which point this is a
+	 *     no-op.
+	 */
+	if (t->zthr_thread == NULL) {
+		t->zthr_thread = thread_create(NULL, 0, zthr_procedure, t,
+		    0, &p0, TS_RUN, minclsyspri);
+	}
 
-	mutex_exit(&t->zthr_lock);
+	mutex_exit(&t->zthr_state_lock);
+	mutex_exit(&t->zthr_request_lock);
 }
 
 /*
  * This function is intended to be used by the zthr itself
- * to check if another thread has signal it to stop running.
+ * (specifically the zthr_func callback provided) to check
+ * if another thread has signaled it to stop running before
+ * doing some expensive operation.
  *
  * returns TRUE if we are in the middle of trying to cancel
  *     this thread.
@@ -319,25 +410,22 @@ zthr_resume(zthr_t *t)
 boolean_t
 zthr_iscancelled(zthr_t *t)
 {
-	boolean_t cancelled;
-
 	ASSERT3P(t->zthr_thread, ==, curthread);
 
-	mutex_enter(&t->zthr_lock);
-	cancelled = t->zthr_cancel;
-	mutex_exit(&t->zthr_lock);
-
+	/*
+	 * The majority of the functions here grab zthr_request_lock
+	 * first and then zthr_state_lock. This function only grabs
+	 * the zthr_state_lock. That is because this function should
+	 * only be called from the zthr_func to check if someone has
+	 * issued a zthr_cancel() on the thread. If there is a zthr_cancel()
+	 * happening concurrently, attempting to grab the request lock
+	 * here would result in a deadlock.
+	 *
+	 * By grabbing only the zthr_state_lock this function is allowed
+	 * to run concurrently with a zthr_cancel() request.
+	 */
+	mutex_enter(&t->zthr_state_lock);
+	boolean_t cancelled = t->zthr_cancel;
+	mutex_exit(&t->zthr_state_lock);
 	return (cancelled);
 }
-
-boolean_t
-zthr_isrunning(zthr_t *t)
-{
-	boolean_t running;
-
-	mutex_enter(&t->zthr_lock);
-	running = (t->zthr_thread != NULL);
-	mutex_exit(&t->zthr_lock);
-
-	return (running);
-}