md: remove flag RemoveSynchronized

rcu is not used correctly here, because synchronize_rcu() is called
before replacing old value, for example:

remove_and_add_spares   // other path
 synchronize_rcu
 // called before replacing old value
 set_bit(RemoveSynchronized)
                        rcu_read_lock()
                        rdev = conf->mirros[].rdev
 pers->hot_remove_disk
  conf->mirros[].rdev = NULL;
  if (!test_bit(RemoveSynchronized))
   synchronize_rcu
   /*
    * won't be called, and won't wait
    * for concurrent readers to be done.
    */
                        // access rdev after remove_and_add_spares()
                        rcu_read_unlock()

Fortunately, there is a separate rcu protection to prevent such rdev
to be freed:

md_kick_rdev_from_array		//other path
				rcu_read_lock()
				rdev = conf->mirros[].rdev
list_del_rcu(&rdev->same_set)

				rcu_read_unlock()
				/*
				 * rdev can be removed from conf, but
				 * rdev won't be freed.
				 */
synchronize_rcu()
free rdev

Hence remove this useless flag and prepare to remove rcu protection to
access rdev from 'conf'.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231125081604.3939938-2-yukuai1@huaweicloud.com

drivers/md/md-multipath.c

@@ -258,15 +258,6 @@ static int multipath_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
 			goto abort;
 		}
 		p->rdev = NULL;
-		if (!test_bit(RemoveSynchronized, &rdev->flags)) {
-			synchronize_rcu();
-			if (atomic_read(&rdev->nr_pending)) {
-				/* lost the race, try later */
-				err = -EBUSY;
-				p->rdev = rdev;
-				goto abort;
-			}
-		}
 		err = md_integrity_register(mddev);
 	}
 abort:

drivers/md/md.c

@@ -9244,44 +9244,19 @@ static int remove_and_add_spares(struct mddev *mddev,
 	struct md_rdev *rdev;
 	int spares = 0;
 	int removed = 0;
-	bool remove_some = false;
 
 	if (this && test_bit(MD_RECOVERY_RUNNING, &mddev->recovery))
 		/* Mustn't remove devices when resync thread is running */
 		return 0;
 
 	rdev_for_each(rdev, mddev) {
-		if ((this == NULL || rdev == this) &&
-		    rdev->raid_disk >= 0 &&
-		    !test_bit(Blocked, &rdev->flags) &&
-		    test_bit(Faulty, &rdev->flags) &&
-		    atomic_read(&rdev->nr_pending)==0) {
-			/* Faulty non-Blocked devices with nr_pending == 0
-			 * never get nr_pending incremented,
-			 * never get Faulty cleared, and never get Blocked set.
-			 * So we can synchronize_rcu now rather than once per device
-			 */
-			remove_some = true;
-			set_bit(RemoveSynchronized, &rdev->flags);
-		}
-	}
-
-	if (remove_some)
-		synchronize_rcu();
-	rdev_for_each(rdev, mddev) {
-		if ((this == NULL || rdev == this) &&
-		    (test_bit(RemoveSynchronized, &rdev->flags) ||
-		     rdev_removeable(rdev))) {
-			if (mddev->pers->hot_remove_disk(
-				    mddev, rdev) == 0) {
-				sysfs_unlink_rdev(mddev, rdev);
-				rdev->saved_raid_disk = rdev->raid_disk;
-				rdev->raid_disk = -1;
-				removed++;
-			}
+		if ((this == NULL || rdev == this) && rdev_removeable(rdev) &&
+		    !mddev->pers->hot_remove_disk(mddev, rdev)) {
+			sysfs_unlink_rdev(mddev, rdev);
+			rdev->saved_raid_disk = rdev->raid_disk;
+			rdev->raid_disk = -1;
+			removed++;
 		}
-		if (remove_some && test_bit(RemoveSynchronized, &rdev->flags))
-			clear_bit(RemoveSynchronized, &rdev->flags);
 	}
 
 	if (removed && mddev->kobj.sd)

drivers/md/md.h

@@ -190,11 +190,6 @@ enum flag_bits {
 				 * than other devices in the array
 				 */
 	ClusterRemove,
-	RemoveSynchronized,	/* synchronize_rcu() was called after
-				 * this device was known to be faulty,
-				 * so it is safe to remove without
-				 * another synchronize_rcu() call.
-				 */
 	ExternalBbl,            /* External metadata provides bad
 				 * block management for a disk
 				 */

drivers/md/raid1.c

@@ -1863,15 +1863,6 @@ static int raid1_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
 			goto abort;
 		}
 		p->rdev = NULL;
-		if (!test_bit(RemoveSynchronized, &rdev->flags)) {
-			synchronize_rcu();
-			if (atomic_read(&rdev->nr_pending)) {
-				/* lost the race, try later */
-				err = -EBUSY;
-				p->rdev = rdev;
-				goto abort;
-			}
-		}
 		if (conf->mirrors[conf->raid_disks + number].rdev) {
 			/* We just removed a device that is being replaced.
 			 * Move down the replacement.  We drain all IO before

drivers/md/raid10.c

@@ -2247,15 +2247,6 @@ static int raid10_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
 		goto abort;
 	}
 	*rdevp = NULL;
-	if (!test_bit(RemoveSynchronized, &rdev->flags)) {
-		synchronize_rcu();
-		if (atomic_read(&rdev->nr_pending)) {
-			/* lost the race, try later */
-			err = -EBUSY;
-			*rdevp = rdev;
-			goto abort;
-		}
-	}
 	if (p->replacement) {
 		/* We must have just cleared 'rdev' */
 		p->rdev = p->replacement;

drivers/md/raid5.c

@@ -8233,15 +8233,6 @@ static int raid5_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
 		goto abort;
 	}
 	*rdevp = NULL;
-	if (!test_bit(RemoveSynchronized, &rdev->flags)) {
-		lockdep_assert_held(&mddev->reconfig_mutex);
-		synchronize_rcu();
-		if (atomic_read(&rdev->nr_pending)) {
-			/* lost the race, try later */
-			err = -EBUSY;
-			rcu_assign_pointer(*rdevp, rdev);
-		}
-	}
 	if (!err) {
 		err = log_modify(conf, rdev, false);
 		if (err)