Re: let md auto-detect 128+ raid members, fix potential race condition

On Sun, 30 Jul 2006 03:56:21 -0300
Alexandre Oliva <aoliva@xxxxxxxxxx> wrote:

I accidentally ran into the 128-devices limit in md.c's
detected_devices. It doesn't seem like a high-enough limit, and I
don't quite see why we wouldn't use a list for this.

Besides, there appears to be a race condition in it:

detected_devices[dev_cnt++] = dev;

won't atomically increment dev_cnt and use its previous value, unless
there's something up the call stack that guarantees mutual exclusion.
I don't see that this is the case.

Previously devices that exceeded the array would be silently
discarded. Now we'll only discard them if we run out of memory, and
we'll report so if we do.

Neil cc'ed.

Before I wrap up, a question on style: does it make sense to use
kzmalloc to allocate this newly-created data structure that contains
only a list_head and a dev_t, where the latter is immediately copied
from another dev_t variable, and then the whole thing is added to a
list? I.e., could any list-checking present or future feature rely on
list_head fields that might hurt if not zero-initialized, or would it
be future-proof to just use kmalloc in this case?

I'd say that if all fields are going to be initialised, kzalloc() shouldn't
be used.

nits:

Index: kernel-2.6.17-1.2462.fc6/drivers/md/md.c
===================================================================
--- kernel-2.6.17-1.2462.fc6.orig/drivers/md/md.c 2006-07-30 01:03:28.000000000 -0300
+++ kernel-2.6.17-1.2462.fc6/drivers/md/md.c 2006-07-30 03:40:54.000000000 -0300
@@ -1435,7 +1435,7 @@ static void unlock_rdev(mdk_rdev_t *rdev
blkdev_put_partition(bdev);
}

-void md_autodetect_dev(dev_t dev);
+int md_register_autodetect_dev(dev_t dev);

Put it in a header file, please.

+int md_register_autodetect_dev(dev_t dev)
{
- if (dev_cnt >= 0 && dev_cnt < 127)
- detected_devices[dev_cnt++] = dev;
+ struct detected_dev_list_t *ldev = kzalloc(sizeof (*ldev), GFP_KERNEL);
+
+ if (!ldev)
+ return -1;

whitespace broke.

}
Index: kernel-2.6.17-1.2462.fc6/fs/partitions/check.c
===================================================================
--- kernel-2.6.17-1.2462.fc6.orig/fs/partitions/check.c 2006-07-30 01:03:34.000000000 -0300
+++ kernel-2.6.17-1.2462.fc6/fs/partitions/check.c 2006-07-30 03:41:02.000000000 -0300
@@ -36,7 +36,7 @@
#include "karma.h"

#ifdef CONFIG_BLK_DEV_MD
-extern void md_autodetect_dev(dev_t dev);
+extern int md_register_autodetect_dev(dev_t dev);
#endif

Again, let's find a header for this.

int warn_no_part = 1; /*This is ugly: should make genhd removable media aware*/
@@ -471,8 +471,10 @@ int rescan_partitions(struct gendisk *di
}
add_partition(disk, p, from, size);
#ifdef CONFIG_BLK_DEV_MD
- if (state->parts[p].flags)
- md_autodetect_dev(bdev->bd_dev+p);
+ if (state->parts[p].flags
+ && md_register_autodetect_dev(bdev->bd_dev+p))
+ printk(KERN_ERR "md: out of memory registering %s%d\n",
+ disk->disk_name, p);
#endif

What happens if CONFIG_BLK_DEV_MD=m?
-
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