linux-kernel - Re: [PATCH] jump_label: Fix anonymous union initialization

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Message-ID: <37023a49-8c13-ba4a-d44c-66c202016cd8@oracle.com>
Date:   Thu, 2 Mar 2017 16:56:55 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Jason Baron <jbaron@...mai.com>
Cc:     peterz@...radead.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org, Chris Mason <clm@...com>
Subject: Re: [PATCH] jump_label: Fix anonymous union initialization

On 03/02/2017 04:42 PM, Steven Rostedt wrote:
> On Thu, 2 Mar 2017 16:07:19 -0500
> Jason Baron <jbaron@...mai.com> wrote:
>
>> On 02/28/2017 11:32 AM, Boris Ostrovsky wrote:
>>> Pre-4.6 gcc do not allow direct static initialization of members of
>>> anonymous structs/unions. After commit 3821fd35b58d ("jump_label:
>>> Reduce the size of struct static_key") STATIC_KEY_INIT_{TRUE|FALSE}
>>> definitions cannot be compiled with those older compilers.
>>>
>>> Placing initializers inside curved brackets works around this problem.
>>>
>>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>>> ---
>>>  include/linux/jump_label.h | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
>>> index 8e06d75..518020b 100644
>>> --- a/include/linux/jump_label.h
>>> +++ b/include/linux/jump_label.h
>>> @@ -166,10 +166,10 @@ extern void arch_jump_label_transform_static(struct jump_entry *entry,
>>>   */
>>>  #define STATIC_KEY_INIT_TRUE					\
>>>  	{ .enabled = { 1 },					\
>>> -	  .entries = (void *)JUMP_TYPE_TRUE }
>>> +	  { .entries = (void *)JUMP_TYPE_TRUE } }
>>>  #define STATIC_KEY_INIT_FALSE					\
>>>  	{ .enabled = { 0 },					\
>>> -	  .entries = (void *)JUMP_TYPE_FALSE }
>>> +	  { .entries = (void *)JUMP_TYPE_FALSE } }
>>>
>>>  #else  /* !HAVE_JUMP_LABEL */
>>>
>>>  
>> (Adding Steve to 'cc)
>>
>> Thanks for the fix.
>>
>> Reviewed-by: Jason Baron <jbaron@...mai.com>
> Funny, Chris pinged me on IRC telling me that jump labels broke with my
> latest tree. And we discovered it was because of anonymous unions and
> he was using an older compiler (4.4 or something). I didn't know how to
> make it work, and we were just going to say "tough, jump labels are not
> for 4.4". Although, didn't goto asm get added into 4.5? Did someone
> backport it to the gcc 4.4 compilers? I believe 4.5 handles anonymous
> unions.

Per https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10676 this was fixed in
4.6. I don't know whether this was ever backported but my gcc 4.4.4 is
definitely not able to deal with those initializers.

As for the fix, it was , in fact, suggested to me by Linus at some point
in the past when a similar issue came up. The only thing here is that we
need to be careful about initialization order.

-boris