On Mittwoch, 1. September 2021 17:41:02 CEST Greg Kurz wrote:
> On Wed, 01 Sep 2021 16:21:06 +0200
>
> Christian Schoenebeck <qemu_...@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
