Fix data race between zil_commit() and zil_suspend()

openzfsonwindows#206 found that it is possible to trip
`VERIFY(list_is_empty(&lwb->lwb_itxs))` when a `zil_commit()` is delayed
by the scheduler long enough for a parallel `zil_suspend()` operation to
exit `zil_commit_impl()`. This is a data race. To prevent this, we
introduce a `zilog->zl_commit_lock` rwlock to ensure that all
outstanding `zil_commit()` operations finish before `zil_suspend()`
begins and that subsequent operations fallback to `txg_wait_synced()`
after `zil_suspend()` has begun.

On `PREEMPT_RT` Linux kernels, the `rw_enter()` implementation suffers
from writer starvation. This means that a ZIL intensive system can delay
`zil_suspend()` indefinitely. This is a pre-existing problem that
affects everything that uses rw locks, so it needs to be addressed in
the SPL.  However, builds against `PREEMPT_RT` Linux kernels are
currently broken due to a GPL symbol issue (#11097), so we can safely
disregard that issue for now.

Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>

include/sys/zil_impl.h

@@ -183,6 +183,7 @@ struct zilog {
 	uint64_t	zl_destroy_txg;	/* txg of last zil_destroy() */
 	uint64_t	zl_replayed_seq[TXG_SIZE]; /* last replayed rec seq */
 	uint64_t	zl_replaying_seq; /* current replay seq number */
+	krwlock_t	zl_commit_lock;	/* protects suspend count */
 	uint32_t	zl_suspend;	/* log suspend count */
 	kcondvar_t	zl_cv_suspend;	/* log suspend completion */
 	uint8_t		zl_suspending;	/* log is currently suspending */

module/zfs/zil.c

@@ -3195,6 +3195,18 @@ zil_commit(zilog_t *zilog, uint64_t foid)
 		return;
 	}
 
+	/*
+	 * The ->zl_commit_lock rwlock ensures that all in-flight
+	 * zil_commit() operations finish before suspension begins and that
+	 * no more begin. Without it, it is possible for the scheduler to
+	 * preempt us right after the zilog->zl_suspend suspend check, run
+	 * another thread that runs zil_suspend() and after the other thread
+	 * has finished its call to zil_commit_impl(), resume this thread
+	 * while zil is suspended. This can trigger an assertion failure in
+	 * VERIFY(list_is_empty(&lwb->lwb_itxs)).
+	 */
+	rw_enter(&zilog->zl_commit_lock, RW_READER);
+
 	/*
 	 * If the ZIL is suspended, we don't want to dirty it by calling
 	 * zil_commit_itx_assign() below, nor can we write out
@@ -3203,11 +3215,13 @@ zil_commit(zilog_t *zilog, uint64_t foid)
 	 * semantics, and avoid calling those functions altogether.
 	 */
 	if (zilog->zl_suspend > 0) {
+		rw_exit(&zilog->zl_commit_lock);
 		txg_wait_synced(zilog->zl_dmu_pool, 0);
 		return;
 	}
 
 	zil_commit_impl(zilog, foid);
+	rw_exit(&zilog->zl_commit_lock);
 }
 
 void
@@ -3472,6 +3486,8 @@ zil_alloc(objset_t *os, zil_header_t *zh_phys)
 	cv_init(&zilog->zl_cv_suspend, NULL, CV_DEFAULT, NULL);
 	cv_init(&zilog->zl_lwb_io_cv, NULL, CV_DEFAULT, NULL);
 
+	rw_init(&zilog->zl_commit_lock, NULL, RW_DEFAULT, NULL);
+
 	return (zilog);
 }
 
@@ -3511,6 +3527,8 @@ zil_free(zilog_t *zilog)
 	cv_destroy(&zilog->zl_cv_suspend);
 	cv_destroy(&zilog->zl_lwb_io_cv);
 
+	rw_destroy(&zilog->zl_commit_lock);
+
 	kmem_free(zilog, sizeof (zilog_t));
 }
 
@@ -3638,11 +3656,14 @@ zil_suspend(const char *osname, void **cookiep)
 		return (error);
 	zilog = dmu_objset_zil(os);
 
+	rw_enter(&zilog->zl_commit_lock, RW_WRITER);
+
 	mutex_enter(&zilog->zl_lock);
 	zh = zilog->zl_header;
 
 	if (zh->zh_flags & ZIL_REPLAY_NEEDED) {		/* unplayed log */
 		mutex_exit(&zilog->zl_lock);
+		rw_exit(&zilog->zl_commit_lock);
 		dmu_objset_rele(os, suspend_tag);
 		return (SET_ERROR(EBUSY));
 	}
@@ -3656,6 +3677,7 @@ zil_suspend(const char *osname, void **cookiep)
 	if (cookiep == NULL && !zilog->zl_suspending &&
 	    (zilog->zl_suspend > 0 || BP_IS_HOLE(&zh->zh_log))) {
 		mutex_exit(&zilog->zl_lock);
+		rw_exit(&zilog->zl_commit_lock);
 		dmu_objset_rele(os, suspend_tag);
 		return (0);
 	}
@@ -3664,6 +3686,7 @@ zil_suspend(const char *osname, void **cookiep)
 	dsl_pool_rele(dmu_objset_pool(os), suspend_tag);
 
 	zilog->zl_suspend++;
+	rw_exit(&zilog->zl_commit_lock);
 
 	if (zilog->zl_suspend > 1) {
 		/*