Re: 9pfs: Twalk crash

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Mittwoch, 1. September 2021 17:41:02 CEST Greg Kurz wrote:
> 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 ?

Yes, I've verified that. In my case an interrupt of Twalk triggered this bug. 
so it was this path exactly:

    v9fs_co_run_in_worker({
        if (v9fs_request_cancelled(pdu)) {
            ...
            break;
        }
        ...
    });

so it was really this break;-ing too far being the root cause of the crash.

> > 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.

Yes, I know, this was my bad.

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

Yep, I'm preparing a patch now.

Best regards,
Christian Schoenebeck