[linux-lvm] Re: [OOPS] full snapshot (with test vfs locking patch for reiserfs snapshots in 11-pre)

[Chris Mason]

On Monday, October 08, 2001 09:57:59 PM -0400 Ed Tomlinson <tomlins at CAM.ORG> wrote:

> Hi Chris
> 
>> Ok, that first patch won't quite fix it, as we can still oops
>> in lvm_snapshot_COW.  This one works better for me:
> 
> Looks like this one has problems too.  Here is the oops I get with it:
>>> EIP; d680b178 <[lvm-mod]_disable_snapshot+10/44>   <=====
> Trace; d680a50a <[lvm-mod]lvm_drop_snapshot+22/94>
> Trace; d680aafc <[lvm-mod]lvm_snapshot_COW+3b4/3f4>
> Trace; d680d3ca <[lvm-mod]lvm_name+8c2/e76>

Hmmm, this should be the same as the bug fixed by the second patch.  We know the snapshot hasn't been released yet when lvm_snapshot_COW is called, so _disable_snapshot should not oops.

We've got a write lock on the lv semaphore, so nobody else should be calling lvm_drop_snapshot on us.  I hate to ask, but are you sure you did an rmmod before the modules_install?

Regardless, we can fix the oops in _disable_snapshot, I just don't see how the locking allows it to happen.  This incremental fix should do it:

--- 0.21/drivers/md/lvm-snap.c Sat, 06 Oct 2001 00:07:22 -0400 
+++ 0.21(w)/drivers/md/lvm-snap.c Mon, 08 Oct 2001 22:39:54 -0400 
@@ -687,6 +694,10 @@
 
 static void _disable_snapshot(vg_t *vg, lv_t *lv) {
 	const char *err;
+	if (!lv->lv_block_exception) {
+		printk(KERN_ERR "%s -- snapshot already disabled\n", lvm_name);
+		return ;
+	}
 	lv->lv_block_exception[0].rsector_org = LVM_SNAPSHOT_DROPPED_SECTOR;
 	if(_write_COW_table_block(vg, lv, 0, &err) < 0) {
 		printk(KERN_ERR "%s -- couldn't disable snapshot: %s\n",