This fixes a regression introduced in commit 9d456654.

aio_co_wake() can only be used to reenter a coroutine that was already
previously entered, otherwise co->ctx is uninitialised and we access
garbage. Using it immediately after qemu_coroutine_create() like in
co_read_response() is wrong and causes segfaults.

Replace the call with aio_co_enter(), which gets an explicit AioContext
parameter and works even for new coroutines.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Tested-by: Kashyap Chamarthy <kchamart@redhat.com>
---
 block/sheepdog.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/sheepdog.c b/block/sheepdog.c
index 1b71fc8..142eb4f 100644
--- a/block/sheepdog.c
+++ b/block/sheepdog.c
@@ -942,7 +942,7 @@ static void co_read_response(void *opaque)
         s->co_recv = qemu_coroutine_create(aio_read_response, opaque);
     }
 
-    aio_co_wake(s->co_recv);
+    aio_co_enter(s->aio_context, s->co_recv);
 }
 
 static void co_write_request(void *opaque)
-- 
1.8.3.1

On 11.04.2017 16:08, Kevin Wolf wrote:
> This fixes a regression introduced in commit 9d456654.
> 
> aio_co_wake() can only be used to reenter a coroutine that was already
> previously entered, otherwise co->ctx is uninitialised and we access
> garbage. Using it immediately after qemu_coroutine_create() like in
> co_read_response() is wrong and causes segfaults.
> 
> Replace the call with aio_co_enter(), which gets an explicit AioContext
> parameter and works even for new coroutines.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> Tested-by: Kashyap Chamarthy <kchamart@redhat.com>
> ---
>  block/sheepdog.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Reviewed-by: Max Reitz <mreitz@redhat.com>

On Tue, Apr 11, 2017 at 04:08:53PM +0200, Kevin Wolf wrote:
> This fixes a regression introduced in commit 9d456654.
> 
> aio_co_wake() can only be used to reenter a coroutine that was already
> previously entered, otherwise co->ctx is uninitialised and we access
> garbage. Using it immediately after qemu_coroutine_create() like in
> co_read_response() is wrong and causes segfaults.
> 
> Replace the call with aio_co_enter(), which gets an explicit AioContext
> parameter and works even for new coroutines.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> Tested-by: Kashyap Chamarthy <kchamart@redhat.com>

Thanks. 

The test evidence was posted on IRC.  But just for archives' sake, I was
here (on Git) when I applied this patch:

    $ git describe
    v2.9.0-rc3-45-gaa388dd

And that was the specific test:

	$ qemu-img convert -t directsync \
		./cirros-0.3.5.qcow2 sheepdog:cirros0.3.5vm

After applying this patch, the segfault is gone, and the `qemu-img convert`
happens successfully.

That was my Sheepdog environment:
https://kashyapc.fedorapeople.org/virt/sheepdog-qemu-corosync.txt

> ---
>  block/sheepdog.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/sheepdog.c b/block/sheepdog.c
> index 1b71fc8..142eb4f 100644
> --- a/block/sheepdog.c
> +++ b/block/sheepdog.c
> @@ -942,7 +942,7 @@ static void co_read_response(void *opaque)
>          s->co_recv = qemu_coroutine_create(aio_read_response, opaque);
>      }
>  
> -    aio_co_wake(s->co_recv);
> +    aio_co_enter(s->aio_context, s->co_recv);
>  }
>  
>  static void co_write_request(void *opaque)
> -- 
> 1.8.3.1
> 

-- 
/kashyap

On 11 April 2017 at 15:08, Kevin Wolf <kwolf@redhat.com> wrote:
> This fixes a regression introduced in commit 9d456654.
>
> aio_co_wake() can only be used to reenter a coroutine that was already
> previously entered, otherwise co->ctx is uninitialised and we access
> garbage. Using it immediately after qemu_coroutine_create() like in
> co_read_response() is wrong and causes segfaults.
>
> Replace the call with aio_co_enter(), which gets an explicit AioContext
> parameter and works even for new coroutines.
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> Tested-by: Kashyap Chamarthy <kchamart@redhat.com>
> ---
>  block/sheepdog.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/block/sheepdog.c b/block/sheepdog.c
> index 1b71fc8..142eb4f 100644
> --- a/block/sheepdog.c
> +++ b/block/sheepdog.c
> @@ -942,7 +942,7 @@ static void co_read_response(void *opaque)
>          s->co_recv = qemu_coroutine_create(aio_read_response, opaque);
>      }
>
> -    aio_co_wake(s->co_recv);
> +    aio_co_enter(s->aio_context, s->co_recv);
>  }
>
>  static void co_write_request(void *opaque)
> --
> 1.8.3.1
>

Applied, thanks.

-- PMM