Re: [PATCH 1/3] Implement generic freeze feature

From: Takashi Sato
Date: Thu Sep 11 2008 - 07:12:36 EST


Christoph Hellwig wrote:
--- 2008-08-29 07:52:02.000000000 +0900
+++ linux-2.6.27-rc5-freeze/fs/block_dev.c 2008-09-05 20:00:29.000000000 +0900
@@ -285,6 +285,8 @@ static void init_once(void *foo)
+ /* Initialize mutex for freeze. */
+ mutex_init(&bdev->bd_fsfreeze_mutex);

Why not just freeze_mutex?

The Linux kernel has already had the name of "freezer" in the part of
power-management. So I named the above mutex "fsfreeze"
(filesystem freeze) to distinguish it from the existent "freezer".
Andrew pointed it out.

struct super_block *freeze_bdev(struct block_device *bdev)
struct super_block *sb;

+ mutex_lock(&bdev->bd_fsfreeze_mutex);
+ if (bdev->bd_fsfreeze_count > 0) {
+ bdev->bd_fsfreeze_count++;
+ sb = get_super(bdev);
+ mutex_unlock(&bdev->bd_fsfreeze_mutex);
+ return sb;
+ }
+ bdev->bd_fsfreeze_count++;

Note that we still have duplication with the bd_mount_sem. I think
you should look into getting rid of it and instead do a check of
the freeze_count under proper freeze_mutex protection.

In the original implementation,
while the filesystem is frozen, subsequent mounts wait for unfreeze
with the semaphore (bd_mount_sem).
But if we replace the semphore with the check of the freeze_count,
subsequent mounts will abort.
I think the original behavior shouldn't be changed, so the existing bd_mount_sem
is better.

@@ -244,6 +274,8 @@ void thaw_bdev(struct block_device *bdev

+ mutex_unlock(&bdev->bd_fsfreeze_mutex);
+ return 0;

Why do you add a return value here if we always return 0 anyway?

I forgot to remove the unneeded return value in above old patch.
But I need to implement a return value in the new patch
because thaw_bdev() needs to return an IO error which occurs
in unlockfs().
Eric pointed it out.

Cheers, Takashi

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