Re: [PATCH] async: Suppress GCC13 false positive in aio_bh_poll()

[Cédric Le Goater]

On 4/24/23 08:33, Thomas Huth wrote:
> On 20/04/2023 22.29, 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 | 14 ++++++++++++++
> >   1 file changed, 14 insertions(+)
> >
> > diff --git a/util/async.c b/util/async.c
> > index 21016a1ac7..856e1a8a33 100644
> > --- a/util/async.c
> > +++ b/util/async.c
> > @@ -164,7 +164,21 @@ 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 "-Wpragmas"
>
> Why do we need to ignore -Wpragmas here?

To handle GCC 8.5 as suggested by Philippe :

  
https://lore.kernel.org/qemu-devel/24d45c41-2f62-76a2-4294-fcfa346c9683@linaro.org/

> > +#pragma GCC diagnostic ignored "-Wdangling-pointer="
> > +#endif
> >       QSIMPLEQ_INSERT_TAIL(&ctx->bh_slice_list, &slice, next);
> > +#if !defined(__clang__)
> > +#pragma GCC diagnostic pop
> > +#endif
> >       while ((s = QSIMPLEQ_FIRST(&ctx->bh_slice_list))) {
> >           QEMUBH *bh;
>
> Anyway:
> Reviewed-by: Thomas Huth <thuth@redhat.com>

Thanks,

C.