Anton Blanchard wrote:

>Hi,
>
>I was unable to boot 2.5-BK on ppc64 and narrowed it down to the fast
>poll patch.  I found:
>
>offsetof(struct poll_list, entries) == 12 but
>sizeof(struct poll_list) == 16
>
>This means pp+1 did not match up with pp->entries. Im not sure what the
>alignment requirements are for a zero length struct (ie is this a
>compiler bug) but the following patch fixes the problem and also changes
>->len to a long to ensure 8 byte alignment of ->entries on 64bit archs.
>  
>
That seems to be the root of the problem: ->entries requires 4 byte 
alignment, while struct poll_list requires 8 byte alignment. Thus the 
compiler aligns entries to a 4-byte boundary, and rounds up 
sizeof(struct poll_list) to 16.

Attached is a small test case that shows the problem on both 64-bit and 
32-bit platforms. I'm not sure if this is a gcc bug: the Compaq C 
compiler on Tru64 and MSVC on Windows generate the same structure layout.

--
    Manfred