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

[Philippe Mathieu-Daudé]

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="