Block script unsafe when starting multiple devices in parallel

Block script unsafe when starting multiple devices in parallel

[Nathan March]

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Hi Guys,

Ran into an issue where losetup was failing, added some logging to it's 
executions to see (first #'s are parent pid / pid):

32435 / 32438 - losetup -f
32436 / 32439 - losetup -f
32435 / 32438 -  Exit code: 0
32435 / 32438 -  Output: /dev/loop/10
32436 / 32439 -  Exit code: 0
32436 / 32439 -  Output: /dev/loop/10
32128 / 32451 - losetup /dev/loop/10 /mnt/xenDisks2/nathanxen2/backup_tmp
31934 / 32452 - losetup /dev/loop/10 /mnt/xenDisks1/nathanxen2/swap

Sometime's losetup seems to be stuck waiting on something else and since 
multiple block devices are brought up in parallel, the block script 
tries to use the same name for multiple loopback devices.

Fix for this is pretty straightforward - should just be letting losetup 
figure out the device names itself, patch is attached.

- Nathan

-- 
Nathan March<nathan@gt.net>
Gossamer Threads Inc. http://www.gossamer-threads.com/
Tel: (604) 687-5804 Fax: (604) 687-5806


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--- block.orig	2011-08-30 10:17:05.663106731 -0700
+++ block	2011-08-30 10:18:49.742946473 -0700
@@ -292,12 +292,12 @@
           done
         fi
 
-        loopdev=$(losetup -f 2>/dev/null || find_free_loopback_dev)
-        if [ "$loopdev" = '' ]
-        then
-          release_lock "block"
-          fatal 'Failed to find an unused loop device'
-        fi
+        #loopdev=$(losetup -f 2>/dev/null || find_free_loopback_dev)
+        #if [ "$loopdev" = '' ]
+        #then
+        #  release_lock "block"
+        #  fatal 'Failed to find an unused loop device'
+        #fi
 
         if LANG=C losetup -h 2>&1 | grep read-only >/dev/null
         then
@@ -305,7 +305,12 @@
         else
           roflag=''
         fi
-        do_or_die losetup $roflag "$loopdev" "$file"
+
+        loopdev=$(losetup -f "$file" --show)
+        if [[ $? -ne 0 ]]; then
+            fatal "Unable to bring up $file - $loopdev"
+        fi;
+        #do_or_die losetup $roflag "$loopdev" "$file"
         xenstore_write "$XENBUS_PATH/node" "$loopdev"
         write_dev "$loopdev"
         release_lock "block"

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Re: Block script unsafe when starting multiple devices in parallel

[Marek Marczykowski]

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On 30.08.2011 19:37, Nathan March wrote:
> Hi Guys,
> 
> Ran into an issue where losetup was failing, added some logging to it's
> executions to see (first #'s are parent pid / pid):
> 
> 32435 / 32438 - losetup -f
> 32436 / 32439 - losetup -f
> 32435 / 32438 -  Exit code: 0
> 32435 / 32438 -  Output: /dev/loop/10
> 32436 / 32439 -  Exit code: 0
> 32436 / 32439 -  Output: /dev/loop/10
> 32128 / 32451 - losetup /dev/loop/10 /mnt/xenDisks2/nathanxen2/backup_tmp
> 31934 / 32452 - losetup /dev/loop/10 /mnt/xenDisks1/nathanxen2/swap
> 
> Sometime's losetup seems to be stuck waiting on something else and since
> multiple block devices are brought up in parallel, the block script
> tries to use the same name for multiple loopback devices.

This should be prevented by locking mechanism (claim_lock /
release_lock), but it isn't working in some cases
(http://lists.xensource.com/archives/html/xen-devel/2011-07/msg00182.html).

I've workarounded it with wrapping this script with flock (in udev rules).

Perhaps it can be done in tools/hotplug/Linux/xen-backend.rules instead
of using claim_lock/release_lock? This is much simpler than
restructuring all hotplug scripts...

-- 
Pozdrawiam / Best Regards,
Marek Marczykowski         | RLU #390519
marmarek at mimuw edu pl   | xmpp:marmarek at staszic waw pl


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