[PATCH -next 6/6] md/md-multipath: remove rcu protection to access rdev from conf

From: Yu Kuai
Date: Sun Oct 15 2023 - 21:28:55 EST


From: Yu Kuai <yukuai3@xxxxxxxxxx>

Because it's safe to accees rdev from conf:
- If any spinlock is held, because synchronize_rcu() from
md_kick_rdev_from_array() will prevent 'rdev' to be freed until
spinlock is released;
- If there is normal IO inflight, because mddev_suspend() will prevent
rdev to be added or removed from array;

And these will cover all the scenarios in md-multipath.

Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
drivers/md/md-multipath.c | 20 ++++++++++----------
1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/drivers/md/md-multipath.c b/drivers/md/md-multipath.c
index aa77133f3188..51c5390c517d 100644
--- a/drivers/md/md-multipath.c
+++ b/drivers/md/md-multipath.c
@@ -32,17 +32,15 @@ static int multipath_map (struct mpconf *conf)
* now we use the first available disk.
*/

- rcu_read_lock();
for (i = 0; i < disks; i++) {
- struct md_rdev *rdev = rcu_dereference(conf->multipaths[i].rdev);
+ struct md_rdev *rdev = conf->multipaths[i].rdev;
+
if (rdev && test_bit(In_sync, &rdev->flags) &&
!test_bit(Faulty, &rdev->flags)) {
atomic_inc(&rdev->nr_pending);
- rcu_read_unlock();
return i;
}
}
- rcu_read_unlock();

pr_crit_ratelimited("multipath_map(): no more operational IO paths?\n");
return (-1);
@@ -137,14 +135,16 @@ static void multipath_status(struct seq_file *seq, struct mddev *mddev)
struct mpconf *conf = mddev->private;
int i;

+ lockdep_assert_held(&mddev->lock);
+
seq_printf (seq, " [%d/%d] [", conf->raid_disks,
conf->raid_disks - mddev->degraded);
- rcu_read_lock();
for (i = 0; i < conf->raid_disks; i++) {
- struct md_rdev *rdev = rcu_dereference(conf->multipaths[i].rdev);
- seq_printf (seq, "%s", rdev && test_bit(In_sync, &rdev->flags) ? "U" : "_");
+ struct md_rdev *rdev = READ_ONCE(conf->multipaths[i].rdev);
+
+ seq_printf(seq, "%s",
+ rdev && test_bit(In_sync, &rdev->flags) ? "U" : "_");
}
- rcu_read_unlock();
seq_putc(seq, ']');
}

@@ -231,7 +231,7 @@ static int multipath_add_disk(struct mddev *mddev, struct md_rdev *rdev)
rdev->raid_disk = path;
set_bit(In_sync, &rdev->flags);
spin_unlock_irq(&conf->device_lock);
- rcu_assign_pointer(p->rdev, rdev);
+ WRITE_ONCE(p->rdev, rdev);
err = 0;
break;
}
@@ -257,7 +257,7 @@ static int multipath_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
err = -EBUSY;
goto abort;
}
- p->rdev = NULL;
+ WRITE_ONCE(p->rdev, NULL);
err = md_integrity_register(mddev);
}
abort:
--
2.39.2