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

[Thomas Huth]

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?

 Thomas