Re: 9pfs: Twalk crash

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Mittwoch, 1. September 2021 14:49:37 CEST Christian Schoenebeck wrote:
> > > And it triggered, however I am not sure if some of those functions I
> > > asserted above are indeed allowed to be executed on a different thread
> > > than main thread:
> > > 
> > > Program terminated with signal SIGABRT, Aborted.
> > > #0  __GI_raise (sig=sig@entry=6) at
> > > ../sysdeps/unix/sysv/linux/raise.c:50
> > > 50      ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
> > > [Current thread is 1 (Thread 0x7fd0bcef1700 (LWP 6470))]
> > 
> > Based in the thread number, it seems that the signal was raised by
> > the main event thread...
> 
> No, it was not main thread actually, gdb's "current thread is 1" output is
> misleading.
> 
> Following the thread id trace, I extended the thread assertion checks over
> to v9fs_walk() as well, like this:
> 
> static void coroutine_fn v9fs_walk(void *opaque)
> {
>     ...
>     assert_thread();
>     v9fs_co_run_in_worker({
>         ...
>     });
>     assert_thread();
>     ...
> }
> 
> and made sure the reference thread id to be compared is really the main
> thread.
> 
> And what happens here is before v9fs_co_run_in_worker() is entered,
> v9fs_walk() runs on main thread, but after returning from
> v9fs_co_run_in_worker() it runs on a different thread for some reason, not
> on main thread as it would be expected at that point.

Ok, I think I found the root cause: the block is break;-ing out too far. The 
following patch should fix it:

diff --git a/hw/9pfs/coth.h b/hw/9pfs/coth.h
index c51289903d..f83c7dda7b 100644
--- a/hw/9pfs/coth.h
+++ b/hw/9pfs/coth.h
@@ -51,7 +51,9 @@
          */                                                             \
         qemu_coroutine_yield();                                         \
         qemu_bh_delete(co_bh);                                          \
-        code_block;                                                     \
+        do {                                                            \
+            code_block;                                                 \
+        } while (0);                                                    \
         /* re-enter back to qemu thread */                              \
         qemu_coroutine_yield();                                         \
     } while (0)

I haven't triggered a crash with that patch, but due to the occasional nature 
of this issue I'll give it some more spins before officially proclaiming it my 
bug. :)

Best regards,
Christian Schoenebeck