On Mon, 6 Sep 2010 21:21:28 +0200
Sam Ravnborg [off-list ref] wrote:

quoted

Comments?

Looks better but can still use a few improvements.
See below.

Thanks for your review and comments....

	Sam

quoted

Thanks,
NeilBrown

commit e4062735c8f7233923df5858ed20f1278f3ee669
Author: NeilBrown [off-list ref]
Date:   Mon Sep 6 14:10:08 2010 +1000

    md: tidy up device searches in read_balance.
    
    We have a pre-increment side-effect in the arg to a macro:
      rcu_dereference
    
    This is poor form and triggers a warning.  Rather than just fix that,
    take the opportunity to re-write the code it make it more readable.
    
    Reported-by: Kulikov Vasiliy [off-list ref]
    Signed-off-by: NeilBrown [off-list ref]

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index ad83a4d..e29e13f 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c

@@ -420,11 +420,13 @@ static void raid1_end_write_request(struct bio *bio, int error)
 static int read_balance(conf_t *conf, r1bio_t *r1_bio)
 {
 	const sector_t this_sector = r1_bio->sector;
-	int new_disk = conf->last_used, disk = new_disk;
-	int wonly_disk = -1;
+	int new_disk = -1;
+	int start_disk;
+	int i;
 	const int sectors = r1_bio->sectors;
 	sector_t new_distance, current_distance;
 	mdk_rdev_t *rdev;
+	int choose_first;

To increase readability the general recommendation is:
1) Sort variable definitions with the longest first.
2) Do not assing variables when they are defined, do that on a separate line
   below the variable definitions.
   With one empty line after variable definitions.

I'm don't really agree with this.  I think declaring related variables
together is much more important that sorting them by length.  I guess it is a
very subjective thing.
And I think initialising at the point of declaration is often a good idea,
though not always.

I've moved 'sectors' up near 'this_sector' but nothing else.

quoted

 
 	rcu_read_lock();
 	/*

@@ -435,54 +437,35 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
  retry:
 	if (conf->mddev->recovery_cp < MaxSector &&
 	    (this_sector + sectors >= conf->next_resync)) {
-		/* Choose the first operational device, for consistancy */
-		new_disk = 0;
-
-		for (rdev = rcu_dereference(conf->mirrors[new_disk].rdev);
-		     r1_bio->bios[new_disk] == IO_BLOCKED ||
-		     !rdev || !test_bit(In_sync, &rdev->flags)
-			     || test_bit(WriteMostly, &rdev->flags);
-		     rdev = rcu_dereference(conf->mirrors[++new_disk].rdev)) {
-
-			if (rdev && test_bit(In_sync, &rdev->flags) &&
-				r1_bio->bios[new_disk] != IO_BLOCKED)
-				wonly_disk = new_disk;
-
-			if (new_disk == conf->raid_disks - 1) {
-				new_disk = wonly_disk;
-				break;
-			}
-		}
-		goto rb_out;
+		choose_first = 1;
+		start_disk = 0;
+	} else {
+		choose_first = 0;
+		start_disk = conf->last_used;
 	}
 
-
 	/* make sure the disk is operational */
-	for (rdev = rcu_dereference(conf->mirrors[new_disk].rdev);
-	     r1_bio->bios[new_disk] == IO_BLOCKED ||
-	     !rdev || !test_bit(In_sync, &rdev->flags) ||
-		     test_bit(WriteMostly, &rdev->flags);
-	     rdev = rcu_dereference(conf->mirrors[new_disk].rdev)) {
-
-		if (rdev && test_bit(In_sync, &rdev->flags) &&
-		    r1_bio->bios[new_disk] != IO_BLOCKED)
-			wonly_disk = new_disk;
-
-		if (new_disk <= 0)
-			new_disk = conf->raid_disks;
-		new_disk--;
-		if (new_disk == disk) {
-			new_disk = wonly_disk;
-			break;
+	for (i = 0 ; i < conf->raid_disks ; i++) {
+		int disk = start_disk + i;
+		if (disk >= conf->raid_disks)
+			disk -= conf->raid_disks;

1) Please comment on the purpose of the for loop

That would be the comment "make sure the disk is operational" ??

2) See comments above aboyt variable definitions

Still disagree - sorry.

quoted

+
+		if (r1_bio->bios[disk] == IO_BLOCKED
+		    || !(rdev = rcu_dereference(conf->mirrors[disk].rdev))
+		    || !test_bit(In_sync, &rdev->flags))

The rather complex expression - which includes a well hidden assignment -
is repeated a few lines later.
Please use a helper function and do not use such hidden assignments.

I've moved the assignment out but I don't agree that a helper function is
needed.
Once must balance the total complexity of the function (which should be kept
low) against the cost of having to go look at a separate piece of code to see
what a helper function actually does.

In this case I think that separating this code out would be 'hiding' rather
than 'abstraction'.

quoted

+			continue;
+
+		if (test_bit(WriteMostly, &rdev->flags)) {
+			new_disk = disk;
+			continue;
 		}
+		new_disk = disk;
+		break;
 	}
 

@@ -491,20 +474,20 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
 	if (this_sector == conf->mirrors[new_disk].head_position)
 		goto rb_out;
 
-	current_distance = abs(this_sector - conf->mirrors[disk].head_position);
+	current_distance = abs(this_sector 
+			       - conf->mirrors[new_disk].head_position);
 
-	/* Find the disk whose head is closest */
+	/* look for a better disk - i.e. head is closer */
+	start_disk = new_disk;
+	for (i = 1; i < conf->raid_disks; i++) {
+		int disk = start_disk + 1;
+		if (disk >= conf->raid_disks)
+			disk -= conf->raid_disks;

See comments about for loop above.
I also cannot see why we suddenly start with 1 where the other
almost identical for loop starts with 0?
If I wonder then someone else will wonder too => comment please.

Before we were finding a working disk.
Now were a finding a better disk.
First is an absolute statement that needs to consider every device,
second is comparative and only needs to consider every other device (... uhm,
that doesn't sound right - I don't mean every second device, I mean every
device that isn't this one).

Suggestions on a comment that would make that clearer?

quoted

 
-	do {
-		if (disk <= 0)
-			disk = conf->raid_disks;
-		disk--;
-
-		rdev = rcu_dereference(conf->mirrors[disk].rdev);
-
-		if (!rdev || r1_bio->bios[disk] == IO_BLOCKED ||
-		    !test_bit(In_sync, &rdev->flags) ||
-		    test_bit(WriteMostly, &rdev->flags))
+		if (r1_bio->bios[disk] == IO_BLOCKED
+		    || !(rdev = rcu_dereference(conf->mirrors[disk].rdev))
+		    || !test_bit(In_sync, &rdev->flags)
+		    || test_bit(WriteMostly, &rdev->flags))
 			continue;

Here the complex expression is repeated - at least almost identical.

The 'almost' is key.
There are two if conditions that are similar but different.  And only two.
Factoring parts out would still leave one of them a little complex and would
split the task of understanding the condition over two separate parts of
program text.
I don't think the benefits of a function out-weigh the costs.

Thanks,
NeilBrown

quoted

 
 		if (!atomic_read(&rdev->nr_pending)) {

@@ -516,11 +499,9 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
 			current_distance = new_distance;
 			new_disk = disk;
 		}
-	} while (disk != conf->last_used);
+	}
 
  rb_out:
-
-
 	if (new_disk >= 0) {
 		rdev = rcu_dereference(conf->mirrors[new_disk].rdev);
 		if (!rdev)

	Sam