On 10/12/2017 07:37 AM, Liang Chen wrote:

The use of the union reduces the size of closure struct by taking advantage
of the current size of its members. The offset of func in work_struct equals
the size of the first three members, so that work.work_func will just
reference the forth member - fn.

This is smart but dangerous. It can be broken if work_struct or the other
structs get changed, and can be a bit difficult to debug.

Signed-off-by: Liang Chen <redacted>

So the objective here is to make sure that struct work_struct and the
anonymous struct remain identical?  I agree that's a potential problem
for future maintenance.

Could we use BUILD_BUG_ON with offsets and sizes to do the same, to get
compile-time checking and avoid doing anything at runtime (I know the
compiler can usually omit the BUG but better to be safe)?  Otherwise a
kernel that triggered this problem would compile, and it'd only be if
someone actually used bcache that it would trigger.

Mike