When mounting SU-enabled mount point, wait until the softdep_flush()

thread started and incremented the stat_flush_threads [1].

Unconditionally wakeup softdep_flush threads when needed, do not try
to check wchan, which is racy and breaks abstraction.

Reported by and discussed with:	glebius, neel
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks

sys/ufs/ffs/ffs_softdep.c

@@ -1377,6 +1377,10 @@ softdep_flush(addr)
 	mp = (struct mount *)addr;
 	ump = VFSTOUFS(mp);
 	atomic_add_int(&stat_flush_threads, 1);
+	ACQUIRE_LOCK(ump);
+	ump->softdep_flags &= ~FLUSH_STARTING;
+	wakeup(&ump->softdep_flushtd);
+	FREE_LOCK(ump);
 	if (print_threads) {
 		if (stat_flush_threads == 1)
 			printf("Running %s at pid %d\n", bufdaemonproc->p_comm,
@@ -1389,7 +1393,7 @@ softdep_flush(addr)
 		    VFSTOUFS(mp)->softdep_jblocks->jb_suspended))
 			kthread_suspend_check();
 		ACQUIRE_LOCK(ump);
-		if ((ump->softdep_flags & FLUSH_CLEANUP) == 0)
+		while ((ump->softdep_flags & (FLUSH_CLEANUP | FLUSH_EXIT)) == 0)
 			msleep(&ump->softdep_flushtd, LOCK_PTR(ump), PVM,
 			    "sdflush", hz / 2);
 		ump->softdep_flags &= ~FLUSH_CLEANUP;
@@ -1421,8 +1425,7 @@ worklist_speedup(mp)
 	LOCK_OWNED(ump);
 	if ((ump->softdep_flags & (FLUSH_CLEANUP | FLUSH_EXIT)) == 0) {
 		ump->softdep_flags |= FLUSH_CLEANUP;
-		if (ump->softdep_flushtd->td_wchan == &ump->softdep_flushtd)
-			wakeup(&ump->softdep_flushtd);
+		wakeup(&ump->softdep_flushtd);
 	}
 }
 
@@ -1471,10 +1474,7 @@ softdep_speedup(ump)
 			    (FLUSH_CLEANUP | FLUSH_EXIT)) == 0) {
 				altump->softdep_flags |= FLUSH_CLEANUP;
 				altump->um_softdep->sd_cleanups++;
-				if (altump->softdep_flushtd->td_wchan ==
-				    &altump->softdep_flushtd) {
-					wakeup(&altump->softdep_flushtd);
-				}
+				wakeup(&altump->softdep_flushtd);
 			}
 			FREE_LOCK(altump);
 		}
@@ -2491,9 +2491,18 @@ softdep_mount(devvp, mp, fs, cred)
 	/*
 	 * Start our flushing thread in the bufdaemon process.
 	 */
+	ACQUIRE_LOCK(ump);
+	ump->softdep_flags |= FLUSH_STARTING;
+	FREE_LOCK(ump);
 	kproc_kthread_add(&softdep_flush, mp, &bufdaemonproc,
 	    &ump->softdep_flushtd, 0, 0, "softdepflush", "%s worker",
 	    mp->mnt_stat.f_mntonname);
+	ACQUIRE_LOCK(ump);
+	while ((ump->softdep_flags & FLUSH_STARTING) != 0) {
+		msleep(&ump->softdep_flushtd, LOCK_PTR(ump), PVM, "sdstart",
+		    hz / 2);
+	}
+	FREE_LOCK(ump);
 	/*
 	 * When doing soft updates, the counters in the
 	 * superblock may have gotten out of sync. Recomputation

sys/ufs/ffs/softdep.h

@@ -1063,6 +1063,8 @@ struct mount_softdeps {
  */
 #define FLUSH_EXIT	0x0001	/* time to exit */
 #define FLUSH_CLEANUP	0x0002	/* need to clear out softdep structures */
+#define	FLUSH_STARTING	0x0004	/* flush thread not yet started */
+
 /*
  * Keep the old names from when these were in the ufsmount structure.
  */