Messages in this thread | | | From | Arnd Bergmann <> | Date | Thu, 2 Jun 2022 14:11:13 +0200 | Subject | Re: mainline build failure due to f1e4c916f97f ("drm/edid: add EDID block count and size helpers") |
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On Thu, Jun 2, 2022 at 1:21 PM Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> wrote: > On 2022/06/02 16:38, Arnd Bergmann wrote: > >> But let's cc the tomoyo and chelsio people. > > > > I think both of them work because the structures are always > > embedded inside of larger structures that have at least word > > alignment. This is the thing I was looking for, and the > > __packed attribute was added in error, most likely copied > > from somewhere else. > > The __packed in "struct tomoyo_shared_acl_head" is to embed next > naturally-aligned member of a larger struct into the bytes that > would have been wasted if __packed is not specified. For example, > > struct tomoyo_shared_acl_head { > struct list_head list; > atomic_t users; > } __packed; > > struct tomoyo_condition { > struct tomoyo_shared_acl_head head; > u32 size; /* Memory size allocated for this entry. */ > (...snipped...) > }; > > saves 4 bytes on 64 bits build. > > If the next naturally-aligned member of a larger struct is larger than > the bytes that was saved by __packed, the saved bytes will be unused.
Ok, got it. I think as gcc should still be able to always figure out the alignment when accessing the atomic, without ever falling back to byte access on an atomic_get() or atomic_set().
To be on the safe side, I would still either move the __packed attribute to the 'list' member, or make the structure '__aligned(4)'.
Arnd
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