Re: 9pfs: Twalk crash

[Greg Kurz <groug@kaod.org>]

On Wed, 01 Sep 2021 16:21:06 +0200
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> 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 

That could explain the breakage indeed since the block you've added
to v9fs_walk() embeds a bunch of break statements. AFAICT this block
breaks on errors... do you know which one ?

> 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);                                                    \

Good.

>          /* 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. :)

Well, this is a pre-existing limitation with v9fs_co_run_in_worker().
This wasn't documented as such and not really obvious to detect when
you optimized TWALK. We've never hit it before because the other
v9fs_co_run_in_worker() users don't have break statements.

But, indeed, this caused a regression in 6.1 so this will need a Fixes:
tag and Cc: qemu-stable.

> 
> Best regards,
> Christian Schoenebeck
> 
>