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

[Juan Quintela]

Cédric Le Goater <clg@kaod.org> 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"
> +#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;

I know, I know.

I like to make fun of the compiler as the next guy.  But it is not
simpler this other change, just put the variable in the heap?

Later, Juan.


>From bb5792a6763a451c72ef5cfd78b09032689f54e5 Mon Sep 17 00:00:00 2001
From: Juan Quintela <quintela@redhat.com>
Date: Tue, 25 Apr 2023 15:19:11 +0200
Subject: [PATCH] Silent GCC13 warning

Gcc complains about putting a local variable on a global list, not
noticing that we remove the whole list before leaving the function.

Signed-off-by: Juan Quintela <quintela@redhat.com>
---
 util/async.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/util/async.c b/util/async.c
index 21016a1ac7..7a8432e9e9 100644
--- a/util/async.c
+++ b/util/async.c
@@ -158,13 +158,17 @@ void aio_bh_call(QEMUBH *bh)
 /* Multiple occurrences of aio_bh_poll cannot be called concurrently. */
 int aio_bh_poll(AioContext *ctx)
 {
-    BHListSlice slice;
+    /*
+     * gcc13 complains about putting a local variable
+     * in a global list, so put it on the heap.
+     */
+    g_autofree BHListSlice *slice = g_new(BHListSlice, 1);
     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;
-- 
2.40.0