Re: [PATCH for-8.0 v2 1/3] async: Suppress GCC13 false positive in aio_bh_poll()

["Cédric Le Goater" <clg@redhat.com>]

On 3/22/23 09:47, Philippe Mathieu-Daudé wrote:
> On 22/3/23 08:11, Thomas Huth wrote:
>> On 21/03/2023 17.16, Cédric Le Goater wrote:
>>> From: Cédric Le Goater <clg@redhat.com>
>>>
>>> GCC13 reports an error :
>>>
>>> ../util/async.c: In function ‘aio_bh_poll’:
>>> include/qemu/queue.h:303:22: error: storing the address of local variable ‘slice’ in ‘*ctx.bh_slice_list.sqh_last’ [-Werror=dangling-pointer=]
>>>    303 |     (head)->sqh_last = &(elm)->field.sqe_next;                          \
>>>        |     ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
>>> ../util/async.c:169:5: note: in expansion of macro ‘QSIMPLEQ_INSERT_TAIL’
>>>    169 |     QSIMPLEQ_INSERT_TAIL(&ctx->bh_slice_list, &slice, next);
>>>        |     ^~~~~~~~~~~~~~~~~~~~
>>> ../util/async.c:161:17: note: ‘slice’ declared here
>>>    161 |     BHListSlice slice;
>>>        |                 ^~~~~
>>> ../util/async.c:161:17: note: ‘ctx’ declared here
>>>
>>> But the local variable 'slice' is removed from the global context list
>>> in following loop of the same routine. Add a pragma to silent GCC.
>>>
>>> Cc: Stefan Hajnoczi <stefanha@redhat.com>
>>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>>> Cc: Daniel P. Berrangé <berrange@redhat.com>
>>> Signed-off-by: Cédric Le Goater <clg@redhat.com>
>>> ---
>>>   util/async.c | 13 +++++++++++++
>>>   1 file changed, 13 insertions(+)
>>>
>>> diff --git a/util/async.c b/util/async.c
>>> index 21016a1ac7..de9b431236 100644
>>> --- a/util/async.c
>>> +++ b/util/async.c
>>> @@ -164,7 +164,20 @@ int aio_bh_poll(AioContext *ctx)
>>>       /* Synchronizes with QSLIST_INSERT_HEAD_ATOMIC in aio_bh_enqueue().  */
>>>       QSLIST_MOVE_ATOMIC(&slice.bh_list, &ctx->bh_list);
>>> +
>>> +    /*
>>> +     * GCC13 [-Werror=dangling-pointer=] complains that the local variable
>>> +     * 'slice' is being stored in the global 'ctx->bh_slice_list' but the
>>> +     * list is emptied before this function returns.
>>> +     */
>>> +#if !defined(__clang__)
>>> +#pragma GCC diagnostic push
>>> +#pragma GCC diagnostic ignored "-Wdangling-pointer="
>>
>> That warning parameter looks like a new one in GCC 13 ?
>> ... so you have to check whether it's available before disabling
>> it, otherwise this will fail with older versions of GCC. I just
>> gave it a try with my GCC 8.5 and got this:
>>
>> ../../devel/qemu/util/async.c: In function ‘aio_bh_poll’:
>> ../../devel/qemu/util/async.c:175:32: error: unknown option after ‘#pragma GCC diagnostic’ kind [-Werror=pragmas]
>>   #pragma GCC diagnostic ignored "-Wdangling-pointer="
>>                                  ^~~~~~~~~~~~~~~~~~~~~
>> cc1: all warnings being treated as errors
> 
> Just curious, does this work? (I don't have a GCC 8.5 handy)
> 
>    #pragma GCC diagnostic push
>    #pragma GCC diagnostic ignored "-Wpragmas"
>    #pragma GCC diagnostic ignored "-Wdangling-pointer="

Yep. That works too.


I like Thomas's proposal too. It only lacks a comment on the compile issue.
Let's see what others have to say about it.

Thanks,

C.