Hi

On Wed, Mar 22, 2023 at 6:42 PM Philippe Mathieu-Daudé <philmd@linaro.org>
wrote:

> On 22/3/23 14:27, Stefan Hajnoczi wrote:
> > 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.
>
> IMO by using #pragmas it is clearer this is a kludge. Also we can
> revert this commit adding the pragmas/comment once the compiler
> are fixed.
>
>
up

fwiw,bBoth solutions look fine to me, as long as there is a comment
explaining it.

-- 
Marc-André Lureau