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

[Stefan Hajnoczi]

On Wed, Mar 22, 2023 at 08:11:37AM +0100, 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
> 
>  Thomas
> 
> What about reworking the code like this:
> 
> diff --git a/util/async.c b/util/async.c
> index 21016a1ac7..b236bdfbd8 100644
> --- a/util/async.c
> +++ b/util/async.c
> @@ -156,15 +156,14 @@ void aio_bh_call(QEMUBH *bh)
>  }
>  /* Multiple occurrences of aio_bh_poll cannot be called concurrently. */
> -int aio_bh_poll(AioContext *ctx)
> +static int aio_bh_poll_slice(AioContext *ctx, BHListSlice *slice)
>  {
> -    BHListSlice slice;
>      BHListSlice *s;
>      int ret = 0;
>      /* Synchronizes with QSLIST_INSERT_HEAD_ATOMIC in aio_bh_enqueue().  */
> -    QSLIST_MOVE_ATOMIC(&slice.bh_list, &ctx->bh_list);
> -    QSIMPLEQ_INSERT_TAIL(&ctx->bh_slice_list, &slice, next);
> +    QSLIST_MOVE_ATOMIC(&slice->bh_list, &ctx->bh_list);
> +    QSIMPLEQ_INSERT_TAIL(&ctx->bh_slice_list, slice, next);
>      while ((s = QSIMPLEQ_FIRST(&ctx->bh_slice_list))) {
>          QEMUBH *bh;
> @@ -191,6 +190,13 @@ int aio_bh_poll(AioContext *ctx)
>      return ret;
>  }
> +int aio_bh_poll(AioContext *ctx)
> +{
> +    BHListSlice slice;
> +
> +    return aio_bh_poll_slice(ctx, &slice);
> +}
> +
>  void qemu_bh_schedule_idle(QEMUBH *bh)
>  {
>      aio_bh_enqueue(bh, BH_SCHEDULED | BH_IDLE);
> 
> Would that work with GCC 13 and be acceptable?

Fine by me. Please add a comment into aio_bh_poll() explaining that this
wrapper function exists to silence the gcc dangling-pointer warning.
Otherwise someone may be tempted to remove the function.

Stefan

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