Finding a rogue src/sys commit with bisection?

Before I totally hose by /usr/src directory, does anyone
have some guidelines on doing a binary search for a rogue
commit in /usr/src/sys?.  Either cam or usb (or acpi?) has
broken the ability to remove a external USB device once it
is plugged into a usb port on my Dell Latitude D530 laptop.
I know that a good kernel can be built with r271273 and
a bad kernel comes from (nearly) top of tree at r274456.

I assume I need to do somthing along the lines

% cd /usr/src/sys
% svn merge -r 274456:272864    (half way point between good and bad)
(build kernel and test)
% cd /usr/src/sys
% svn revert -R .
(assume 272864 builds working kernel)
% svn merge -r 274456:273660   (1/2 point between 272864 and 274456).

Rinse and repeat.

-- 
Steve