linux-kernel - Re: [PATCH v2 1/3] hpsa: use __packed on individual structs, not header-wide

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Date:   Tue, 30 Mar 2021 09:30:44 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Sergei Trofimovich <slyfox@...too.org>
Cc:     "Martin K. Petersen" <martin.petersen@...cle.com>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Don Brace <don.brace@...rochip.com>,
        linux-ia64@...r.kernel.org, storagedev@...rochip.com,
        linux-scsi <linux-scsi@...r.kernel.org>, jszczype@...hat.com,
        Scott Benesh <scott.benesh@...rochip.com>,
        Scott Teel <scott.teel@...rochip.com>, thenzl@...hat.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] hpsa: use __packed on individual structs, not header-wide

On Tue, Mar 30, 2021 at 9:22 AM Sergei Trofimovich <slyfox@...too.org> wrote:
>
> Some of the structs contain `atomic_t` values and are not intended to be
> sent to IO controller as is.
>
> The change adds __packed to every struct and union in the file.
> Follow-up commits will fix `atomic_t` problems.
>
> The commit is a no-op at least on ia64:
>     $ diff -u <(objdump -d -r old.o) <(objdump -d -r new.o)

This looks better to me, but I think it still has the same potential bug in
the CommandList definition. Moving from #pragma to annotating the
misaligned structures as __packed is more of a cleanup that could
be done separately from the bugfix, but it does make it a little more
robust.

>  #define HPSA_INQUIRY 0x12
>  struct InquiryData {
>         u8 data_byte[36];
> -};
> +} __packed;

Marking this one and a few others as __packed is a bit silly, but
also obviously harmless, and closer to the original version, so that's
ok.

> @@ -451,7 +452,7 @@ struct CommandList {
>         bool retry_pending;
>         struct hpsa_scsi_dev_t *device;
>         atomic_t refcount; /* Must be last to avoid memset in hpsa_cmd_init() */
> -} __aligned(COMMANDLIST_ALIGNMENT);
> +} __packed __aligned(COMMANDLIST_ALIGNMENT);

You are still marking CommandList as __packed here, which is
what caused the original problem. Please don't mark this one
as __packed at all. If there are individual members that you want
to be misaligned inside of the structure, you could mark those
explicitly.

      Arnd