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 <c...@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 <stefa...@redhat.com>
Cc: Paolo Bonzini <pbonz...@redhat.com>
Cc: Daniel P. Berrangé <berra...@redhat.com>
Signed-off-by: Cédric Le Goater <c...@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