linux-kernel - Re: next-20170217 boot on POWER8 LPAR : WARNING @kernel/jump

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Message-ID: <877f4in5tn.fsf@concordia.ellerman.id.au>
Date:   Wed, 22 Feb 2017 16:38:12 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Jason Baron <jbaron@...mai.com>,
        Sachin Sant <sachinp@...ux.vnet.ibm.com>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-next@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        linuxppc-dev@...abs.org
Subject: Re: next-20170217 boot on POWER8 LPAR : WARNING @kernel/jump_label.c:287

Jason Baron <jbaron@...mai.com> writes:

> On 02/20/2017 10:05 PM, Sachin Sant wrote:
>>
>>> On 20-Feb-2017, at 8:27 PM, Jason Baron <jbaron@...mai.com
>>> <mailto:jbaron@...mai.com>> wrote:
>>>
>>> Hi,
>>>
>>> On 02/19/2017 09:07 AM, Sachin Sant wrote:
>>>> While booting next-20170217 on a POWER8 LPAR following
>>>> warning is displayed.
>>>>
>>>> Reverting the following commit helps boot cleanly.
>>>> commit 3821fd35b5 :  jump_label: Reduce the size of struct static_key
>>>>
>>>> [   11.393008] ------------[ cut here ]------------
>>>> [   11.393031] WARNING: CPU: 5 PID: 2890 at kernel/jump_label.c:287
>>>> static_key_set_entries.isra.10+0x3c/0x50
>>>
>>> Thanks for the report. So this is saying that the jump_entry table is
>>> not at least 4-byte aligned. I wonder if this fixes it up?
>>>
>>
>> Yes. With this patch the warning is gone.
>
> Hi,
>
> Thanks for testing. We probably need something like the following to 
> make sure we don't hit this on other arches. Steve - I will send 4 
> separate patches for this to get arch maintainers' acks for this?

What's the 4 byte alignment requirement from?

On 64-bit our JUMP_ENTRY_TYPE is 8 bytes, should we be aligning to 8
bytes?

> diff --git a/arch/powerpc/include/asm/jump_label.h 
> b/arch/powerpc/include/asm/jump_label.h
> index 9a287e0ac8b1..f870a85bac46 100644
> --- a/arch/powerpc/include/asm/jump_label.h
> +++ b/arch/powerpc/include/asm/jump_label.h
> @@ -24,6 +24,7 @@ static __always_inline bool arch_static_branch(struct 
> static_key *key, bool bran
>          asm_volatile_goto("1:\n\t"
>                   "nop # arch_static_branch\n\t"
>                   ".pushsection __jump_table,  \"aw\"\n\t"
> +                ".balign 4 \n\t"

Can you line those up vertically?

(That may just be an email artifact)

>                   JUMP_ENTRY_TYPE "1b, %l[l_yes], %c0\n\t"
>                   ".popsection \n\t"
>                   : :  "i" (&((char *)key)[branch]) : : l_yes);
> @@ -38,6 +39,7 @@ static __always_inline bool 
> arch_static_branch_jump(struct static_key *key, bool
>          asm_volatile_goto("1:\n\t"
>                   "b %l[l_yes] # arch_static_branch_jump\n\t"
>                   ".pushsection __jump_table,  \"aw\"\n\t"
> +                ".balign 4 \n\t"
>                   JUMP_ENTRY_TYPE "1b, %l[l_yes], %c0\n\t"
>                   ".popsection \n\t"
>                   : :  "i" (&((char *)key)[branch]) : : l_yes);
> @@ -63,6 +65,7 @@ struct jump_entry {
>   #define ARCH_STATIC_BRANCH(LABEL, KEY)         \
>   1098:  nop;                                    \
>          .pushsection __jump_table, "aw";        \
> +       .balign 4;                              \
>          FTR_ENTRY_LONG 1098b, LABEL, KEY;       \
>          .popsection
>   #endif

Otherwise that looks fine assuming 4 bytes is the correct alignment.

cheers