The attached patch will correct the ordering of trace output between 
request queue insertions (I) and unplug calls (U). Right now the insert 
precedes the unplug, which just isn't right:

 65,128  0        1    67.699868965  7882  Q   R 0 + 1 [aiod]
 65,128  0        2    67.699876462  7882  G   R 0 + 1 [aiod]
 65,128  0        3    67.699878286  7882  P   W [aiod]
 65,128  0        4    67.699880491  7882  I   R 0 + 1 [aiod]
 65,128  0        5    67.699887589  7882  U   R [aiod] 1
 65,128  0        6    67.699898317    54  D   R 0 + 1 [kblockd/0]
 65,128  2      153    67.700126590     0  C   R 0 + 1 [0]

With the patch provided the unplug comes first:

 65,128  3        1     0.000000000  7045  Q   R 0 + 1 [aiod]
 65,128  3        2     0.000002295  7045  G   R 0 + 1 [aiod]
 65,128  3        3     0.000002617  7045  P   W [aiod]
 65,128  3        4     0.000003685  7045  U   R [aiod] 1
 65,128  3        5     0.000004107  7045  I   R 0 + 1 [aiod]
 65,128  3        6     0.000009491    57  D   R 0 + 1 [kblockd/3]
 65,128  2        1     0.000232447     0  C   R 0 + 1 [0]

Jens: If you agree, the patch can be applied to your plug branch for 
git://git.kernel.dk/data/git/linux-2.6-block.git

Thanks,
Alan